2*2=4, so that would be a quadruple, I guess.

Well, this is exactly what I have always been saying. Many previous studies tend to ignore the effect of heat generated by incandescents on your heating bill. In contrast, this study does mention this effect (simple thermodinamics as you correctly state it), but I don't believe their figures.
Their study says that assuming 75% of the total time the incandescent light bulb is used, is during periods where central heating is also used, and that central heating is fuled by natural gas, the total amount of CO2 emissions saved on your GAS (heating) bill will only by about 17 kg over the whole 25000 years.
I think their math doesn't add up, since it should not be too much lower than just 75% of the CO2 emissions of the electrical energy used to light the bulb (again, relatively simple thermodynamics). Searching on the internet a little, one finds that 1 kWh of heat from natural gas produces roughly 200gr of CO2. Considering all the energy going into an incandescent bulb is converted into heat (that little fraction that is emitted as light eventually also generates heat - simple thermodynamics), that means that this 40Watt light bulb is generating 25000*0.04*0.75 = 750 kWh of heat over its 25000 hour life-span (well, in the study that's actually 25 light bulbs at 1000h each).
That yields approximately 150kg of CO2 saved by the bulb(s)... sounds much more like it.

And this result is without considering the fact that central heating also has losses (through exhaust, ventilation, etc... which yields less than 100% efficiency), so saved emissions should be even higher!

They actually briefly mention this point in the study, so it is at least being considered (RTFA).
Although I do not entirely agree with some assumptions made and the last calculation about heat.
You also forget to mention the opposite possibility: excess heat being benefical in colder places. The study mentions an example where they dismiss the heating effect of the incandecent bulb under conditions where the bulb is used 75% of the time when heating is needed, as saving only 17 kg of CO2. I have made some calculations, and being very pessimistic, assuming your central heating has 100% efficiency, you should still save some 150 kg of CO2 over the 25000 hour lifecycle. That is not being insignificant anymore, but the study just dismisses it.

Please mod this one up! Oregano is indeed an excelent frontend (although with some rough edges and a few bugs) that whould run on Linux, MacOSX and probably also Windows, since it's built on GTK+.
It is quite easy and intuitive to use, works much better than gEDA, but needs some getting used to working around it's glitches, specially to discover a few tricks about how to work easily with any external spice subcircuit.
Simulation can be done through berkeley-SPICE, ngspice or GnuCAP.
It's a shame this is the only post so far mentioning it.

Why not? Of course there is! Where does all the electric energy go that your PC consumes? I'll split it out for you:

- Most part is converted into heat directly in wire resistance and switching losses of transistors (in power supply, processor, logic, etc).

- Then there's light from the LED's and your monitor. But light ends up being transformed into heat ALWAYS, whether it is inside the room you're in, or outside (that portion of light that escapes through your windows).

- Then there's acoustic energy, from vibrating hdd heads, fans, etc... but that also ends up as heat being absorbed by walls and whatever (except maybe for the tiny fraction of noise that escapes the room).

- And finally there's moving air (from the fans) which ends up converted into heat through friction.

Conclusion: your PC is as efficient as an electric heater in generating heat... difference is that your PC can do something useful (other than generating heat) in the meantime (like downloading pr0n ;-), at no cost in terms of heat-conversion efficeincy.
Please note that I never intended to say that it is a good idea to leave your PC powered-on when not used. I merely wanted to make a point that figures stating the supposed amount of wasted energy are not quite realistic.... but it still is a waste of energy.

Leaving PC's on when they are not used is most probably a terrible waste, but I suspect that numbers about losses due to this are probably not very accurate. At least I have never seen evidence that those calculations take into account the simple fact that energy never dissapears, it only changes nature:

This way unused PC's basically transform electrical energy into heat... with 100% efficiency (!). In many parts of the world however, during important parts of the year, heating is necessary. Heating costs a certain amount of energy, whether it comes from burning gas or oil directly or from electricity is just a matter of a difference in price (heat generated from electricity is probably more expensive). Of course you'd say that leaving the heating on during the night in a building that is only used during the day is also a waste, but take into consideration that (big) buildings do have quite a considerable thermal mass, so if you keep it warmer over night, the next day you still need less energy to heat it up again.

Conclusion: when the heating is actively used, leaving your PC (or light-bulb, stand-by transformer or whatever) on when not used, will still save you money on the gas bill (but cost you more on electricity of course). The overall balance is still for a loss of course, but in some situations, a significantly smaller loss than many people tend to think.

The same idea is true for energy saving light bulbs, btw, but that's for a different discussion.