It looks like Microsoft is catching up with most of the Linux distributions I’ve ran over the last few years, in at least one way. Vista will have the capability for users to run as a “non-priveledged” user instead of admin, but still be able to perform admin tasks without logging off and back on as admin.
This feature has been around forever on UNIX (and UNIX-like) operating systems at the commandline. The “su” command would let you run as root whenever you needed it. I know Windows has had the “runas” command for a while now too, but it is a little more of a pain to use. Most Linux distributions also have a feature in their windowing environment, like KDE or GNOME where if you try to do something that requires root, it prompts you for the root password and continues. It looks like Windows Vista will have this feature too, finally!