First off, forget Hitler's Munich Olympics broadcast, that's way to new. The most interesting thing about Earth is roughly half a billion years old, and that's its "unnatural" atmosphere. Our atmosphere shouts, "Life!" like nothing else. The stuff in our air just doesn't cohabit from ordinary chemical processes - it has to be maintained. Not as old, but still older than Hitler's broadcast is the sustained presence of pollutants in the atmosphere. This might suggest, "intelligent, if immature/foolhardy life."
We can almost see this kind of stuff with Kepler, though to get to this level of detail we use several instruments in parallel - Kepler is the first-weeder. We're nowhere near having interstellar technology, so any race that does will likely have commensurate technologies in other areas as well. Most notably, if you're going to travel far, you want to know which direction to go, and as much about your destination as you can. They would have tools that make Kepler look like a child's toy. They would know how interesting Earth is. Where that ranks us with respect to other planets in another question, but I'll bet it's not as bleak a prospect as some say.
Personally I think the presence of us on Earth has to do with it's "sufficiently interesting history", including the collision that formed the moon, several asteroid/comet strikes like the dinosaur killer, etc. Not to mention plate tectonics, the magnetic field that keeps the solar wind from blowing our atmosphere away, etc. Like I said, I think Earth would be on the short-list.
By the same token, I also think they would observe. Our society and existence are fragile enough, one big kick could easily topple the whole mess. Imagine a preemptive strike by one power to prevent another power from getting "the advantages of alien technology," etc. We're also pretty darned "memetically susceptible," and even allowing an alien idea to reach us might upset the apple cart.
Or as an alternative, perhaps the Catholic Church was right, and Galileo (and Copernicus) were wrong. If not the physical center of the universe, if we're all there is, perhaps the Earth is the philosophical center of the universe.
So:
1 - We're all there is, perhaps to become the Progenitors, perhaps not.
2 - There is other life, hasn't gotten here yet, may not bother, may not be able.
3 - There is other life, observing us, careful to remain unknown - the Prime Directive.
4 - There is other life, getting ready to invade/destroy us.
5 - There is other life, in contact only with the Illuminati and Club of Rome.
Personally I'd prefer option 3. Option 2 is equally likely. Option 1 is rather sad. Options 4 and 5 are IMHO silly.
