Extrasolar planets have a whole other definition that was established just for them already. If you've got a problem with that one, arguing about the 2006 definition of solar planets instead is kind of pointless. The extrasolar one is labeled as a "working" definition anyway so I doubt there's much controversy over the fact that it's not very good yet.
The Stern-Levison parameter of an object is unaffected by whether it's being bombarded, it's only meant to show the object's orbit-clearing capability. Unsurprisingly, now that billions of years have passed since our solar system formed, objects with high orbit-clearing capability happen to have orbits that are clear of other debris. But Earth would have had the same Stern-Levison parameter four and a half billion years ago as it does now, so by that measure it's quite clear what it would have been.
It's probably not a productive exercise to try to figure out the exact moment when a single grain of sand lands on a dwarf planet and makes it into a planet. The current definition has a clear bimodal distribution to it which is probably the best we can hope for for something like this.
An object can be "cleared" from another object's orbit without actually leaving the physical proximity of the other object. It suffices that the dynamics of the situation are such that there's no chance that the two objects will ever collide. This means that both moons and objects in resonant orbits (eg Jupiter's Trojan asteroids, which are in a 1:1 resonant orbit, and Pluto, which is in a 3:2 resonance with Neptune) are considered "cleared" from a planet's orbit.
There actually are more rigorous ways of defining and measuring the degree to which a planet has cleared its orbit, or the degree to which a planet is capable of clearing its orbit. You can see a good summary of them here: http://en.wikipedia.org/wiki/Clearing_the_neighbourhood. I expect the Stern-Levison method is the main one the IAU had in mind since the paper describing it was presented to the IAU back in 2000. It's a mathematical formula that's applicable to any planet that you know the mass and orbital period of and gives an objective value for how capable it is of clearing its orbital neighborhood. Plotting the Stern-Levison parameters for the major bodies of the Solar System shows an orders-of-magnitude gap between the planets and the dwarf planets.
The IAU already has a separate working definition for what constitutes an "extrasolar planet", BTW: http://www.dtm.ciw.edu/boss/definition.html. It says that "The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System." So probably the 2006 IAU definition of Sun-orbiting planets already applies to extrasolar ones even though the 2006 definition explicitly required planets to orbit the Sun. It just doesn't matter much yet because we generally can't detect anything small enough around other stars to be considered borderline.
as for your unstable balance of o2 and carbon, thats pretty much earth, right now. i can walk into most any nonwater environment on earth and start an inferno by myself if i wanted to. and yet our biosphere has lasted a plenty long time, mainly because the biosphere maintains the balance. it would be maintained biologically the same way on venus
I think you're underestimating the magnitude of the difference here, and just how reactive that much pure oxygen would be. The Apollo 1 fire happened because the capsule was filled with slightly more than one atmosphere of pure oxygen, which made anything remotely flammable turn into a blowtorch the moment it ignited. This would be more than an order of magnitude worse than that. And the planet wouldn't cool down appreciably until after you'd got rid of the atmosphere, so it'd still be hot enough to melt lead while you're starting to introduce both high-pressure oxygen and a fuel source to the environment. Photosynthetic life might help maintain a livable environment after you've made it livable, but it's not going to get there by itself - no way no how. Sagan himself retracted the idea in his book Pale Blue Dot.
One of the proposals I've seen that seems much more plausible is to refine calcium and magnesium metal from extravenusian sources such as the Moon or asteroids, and bombard the planet with ingots of it. Calcium and magnesium metal can react with carbon dioxide to form solid carbonates. You'd still need an enormous amount of it, though, and I'm not 100% sure on whether even inorganic carbonates would be stable at Venusian temperatures.
Alternately, you could perhaps colonize Venus as-is using aerostat cities. At an altitude of 50 kilometers above Venus' surface the atmospheric pressure is about the same as sea level on Earth and the temperature is in the 0-50 degree C range, the most Earthlike conditions you can find anywhere in the solar system outside of Earth itself. What's more, an Earthlike oxygen-nitrogen mix is lower density than Venus' carbon dioxide atmosphere, so your balloon city would be able to float using nothing but its own internal air as a lifting gas. A spiffy idea, IMO.
but i always thought venus was a better target for terraforming. its easier to subtract out of venus' atmosphere than put in mars' atmosphere what isn't there. i didn't say EASY, i said EASIER. some sort of genetically engineered bug that sequesters all of the CO2 and H2SO4, and permanently precipitates it out, preferably leaving O2 and H2O. something that could live on top of the clouds and in them.
Actually, no, it's way harder to terraform Venus than it is to terraform Mars. The "just introduce algae" idea was proposed in 1961 by Carl Sagan, before the full extent of just how awful Venus' atmosphere was was fully appreciated. Venus has 90 atmospheres worth of carbon dioxide, and pretty much no available hydrogen. If you want to convert carbon into organic molecules, you need to have hydrogen - carbon alone is not sufficient. But if by some chance you did somehow convert 90 atmospheres worth of carbon dioxide into carbon and oxygen, what you'd wind up with is a furnace-hot planet with 60 atmospheres of pure oxygen and a layer of flammable carbon several hundred feet thick. This is not a stable situation, it'll go right back to the way it is now very quickly and spectacularly (though since the carbon would have been burning as fast as it's produced you'd never get such an extreme disequilibrium in real life). The permanent sequestration of all that carbon dioxide will require the addition of more material to the planet's atmosphere from the outside than would be required to give Mars a whole new atmosphere from scratch.
Furthermore, once you've given Venus an Earthlike atmosphere, there's another issue to consider; Venus has a rotation that's 243 Earth days long. Night lasts for 122 days on Venus. Without its ultra-dense atmosphere to convey heat around it's going to get extremely cold in the dark. We'll have to come up with a whole new ecology to endure those conditions and it doesn't sound all that fun for human inhabitants.
(Practically speaking I imagine it'll come down to something entirely different, such as taking up there offer of low resolution images in order to avoid the risk of a personal tragedy of a lawsuit).
This probably isn't a possible compromise. The person they're threatening to sue, Dcoetzee, is just an individual Wikipedia editor. He can't "take up an offer" on the behalf of Wikipedia, or delete the high-resolution pictures that he's already uploaded. That's up to the Wikimedia foundation, who have declined the gallery's many previous demands to do so (since under US law such demands are completely groundless).
I suspect that what the gallery really wants is to make an example of Dcoetzee and establish precedent within UK law. A compromise seems unlikely to achieve that.
Other posters have already mentioned erosion via the expansion and contraction of the monthly day/night cycle's heating and cooling, and erosion by micrometeors. There's also moonquakes and electrostatic levitation of moon dust that come to mind as other natural sources of erosion.
On top of all that, there's artificial sources of erosion. Bear in mind that the footprint was made at the base of a ladder that a couple of astronauts spent hours coming and going from; it probably got stepped on a few times. And then the lander took off again by firing a powerful rocket engine, directly blasting the area with high-velocity gases. You can see in a video of Apollo 17's lander launch that quite a lot of dust and debris gets blown about in the process. http://www.youtube.com/watch?v=AXs4tncQcAE
But frankly, even if that first footprint was still magically pristine, I don't think returning there and putting down new footprints would somehow "ruin" the historical significance. It would add to the historical significance. The site would no longer be just the site of the first manned lunar landing, it'd be the site of the first manned lunar landing and the first return to the site of the first manned lunar landing. That's pretty neat too.
"It's a dog-eat-dog world out there, and I'm wearing Milkbone underware." -- Norm, from _Cheers_