That's what I have done with my new pc. One drive for XP and one for Vista. Then use an 'icy box' sata drive swappable enclosure thingy to swap the drives around as needed. I'm still using XP drive 90% of the time because of some weird old work programs that don't work in Vista. I'm just using vista for games and playing around with stuff.
The reason I didn't go for a dual boot system is because of the one problem that is widely reported, in that restore points get wiped out when swapping between os's, specially when you boot into XP and it wipes out any Vista restore points. There are some workarounds though that some people say work and some say they don't.
Anyway, I'm happy with my setup and at the end of the day when Vista does everything I need, I'll simply wipe the XP drive and use both drives internally and dump the hot-swap system.
Boo!