There are plenty of sites out there that compile wallpapers, but most have a very low signal:noise ratio. I don't want to deal with so much 'rough' to find a couple 'diamonds'. Are there places that have a generally high level of quality?
Mantia (who now works for Apple) has a nice selection of geeky iPhone/iPad wallpapers. Some of his stuff is featured at the Iconfactory, along with some other wallpapers by Dave Brasgalla, Gedeon Maheux and others.