We can all agree that the wallpapers Vista ships with are jaw dropping.
However, if you are starting to get bored and desperately seeking more, there are good news.
Thanks to Hamad Darwish, the talented artist who had some of his photographs put on as Vista's wallpapers. You may download the photographs that didn't make it to RTM right from his page.
http://www.hamaddarwish.com/content/index_2.html
Also, here is a quick tip. I use ScrollWall, a free utility that allows me to rotate wallpapers based on time.
http://www.vcsoftwares.com/sw.html