I caught this via Steve Clayton (who caught it via Jeff Sandquist showing off his home) - Stardock has a really neat application called LogonStudio that allows you to change the background of your logon screen in Windows Vista. I absolutely love the space background.
LogonStudio is available as a free download and also allows you to create your own backgrounds as well as download logon backgrounds via Stardock"s WinCustomize Gallery too. Folks who create their own backgrounds can submit them to the WinCustomize Gallery. Give it a try. Let me know what you think.