There is carrier neutral colocation - why not have carbon neutral server farms? TMC Green Technology World interviews Rackspace as a follow-up to their customer survey about making their data centers green.
Rackspace is focusing on company conservation, customer offerings and employee education. They are also co-marketing efforts from their partner NativeEnergy. NativeEnergy is a pretty interesting company and has a good list of big name companies as members. They have a number of renewable energy sources that aren't always the ones making the news. My favorite is "remooable energy". :)
Check out the TMC article here