This is a great idea - I was looking for ways to cut down on our companies business carbon footprint and found carbonfreesearch.com
They started out in the charity game also has a huge program for the general internet user

Now White Dot Box have gone green - Done my homework on them and they look 100% legit plus backed by Yahoo (IMHO great move from Yahoo to back this)

If this takes off its a great way to offset your business carbon by doing what we all do search the web

I think this is great idea but I am just one man be great to hear your thoughts