I'd much rather grab wallpaper for the iPad from this thread than going to someone else's website to get it. First off, I'm here already. Secondly, I know if its posted here it means someone is likely already using it on their iPad. Third, I tend to respect the users on this forum for digging up the best of the best and I don't have to sift through some other site to find them. And finally, the UI of this site is very easy to navigate versus some of the dedicated sites listed in this thread.
Interfacelift or whatever, for example, has a very amateurish user interface.