Ask.com, Google and Yahoo! all sported logos for Earth Day yesterday. I did not see special logos at Live.com or even Dogpile (but I may have missed them). Here they are:
Yahoo! had this cute animation, which I took a screen cast of and put on YouTube. The video is pretty stretched out, so please keep that in mind.
Here is the Flash file from Yahoo (they may move the file off the server in the future):