Microsoft has set a lot of restrictions on Vista, but if this latest one holds true, it could mean the difference between upgrading or not for some people. Apparently if you buy an upgrade version of Windows Vista for your XP machine, Windows Vista will invalidate your XP key (so you won't be able to set up a dual-boot option nor will you be able to use that version of XP on another machine). Not only that, but if you ever uninstall Vista, you won't be able to fall back on your copy of XP anymore. Nice way to entice the upgraders, MS.
Vista Upgrade Invalidates Your XP Key [Tom Coyote]