FYI, OS X does this pretty well and and it sounds like Vista will too. OS X allows for "smart folders" which are just folders created on the fly, based on whatever tag you're searching for. Definitely a very helpful aspect of the system. If you're judicious with your tags then suddenly it doesn't really matter where you store things because they'll always show up in your smart folder for that particular tag or set of tags.

The free utility called Quicksilver expedites the process of tagging files in a few ways, all of them helpful. I swear, I fall more in love with my Powerbook every day. :-)