I've worked on a couple of environmental focused websites such as VerdantConcepts and Greener Earth Maps. A concern has always been the benefit/environmental cost ratio of running servers 24/7 that use up power and resources. This has been a big issue in persistant worlds such as Second Life where every acre of digital world has a real-world cost due to maintaining the simulation, even if no one is there!.
Therefore, it was a happy surprise when one of the hosting companies I use (such as this blog), Dreamhost, announced they are carbon neutral. What this means is they've purchased certificates from a carbon offset provider. You can be carbon neutral by planting trees, using renewable energies, use carbon sequestration techniques, or the easy (yet effective) method of buying carbon offsets from an organization that will do this for you.
The internet, and computing usage, has a huge, and growing, impact on the environment. Companies create power stations just to power large internet companies and their hosting requirements. Therefore it is important to understand this impact and alleviate it through any of those means mentioned above.
In some ways, it is buying yourself the "Green" label, but in the end the effect is the same. You're putting your money where the need is, saving the environment.
So the question you should all be asking yourselves: "Is my hosting provider carbon neutral?" If not, make them, or move to one that is.
Update: for the low-down on how carbon offsets reallythis informative-info-graphic from Salon. (thanks to Sean Gillies for being the sleuthy detective in finding this gem via Reddit).