I have Vista and XP set as dual boot on my machine with no problems.
HOWEVER, I have noticed that on most of the forums I'm a member of that people are having a LOT of issues surrouding the install of Vista on their existing machines and some are even losing data over it.
EASIEST way to protect yourself is to download VMWare or Virtual PC and install it that way. I would suggest VMWare as they just added a base for Vista that's in Beta and VERY stable.
This is just a friendly suggestion to help those who aren't too sure about what they're doing when it comes to dual booting or installing Vista. Doing it via a Virtual Computer is SAFE and easy to work with.