I've been running Vista for a few builds now, most recently getting updated to Beta 2. It's fairly common for me to need to mount ISO images of CD's/DVD's. My favorite, Daemon Tools hasn't worked in recent builds so I was forced to look elsewhere. It turns out that
elby Virtual CloneDrive is free, and works great in Vista. It's possible that Daemon Tools now works in Beta 2, but their last release that promised Vista support never installed properly so I felt a bit let down. If you need to mount an ISO in Vista, give it a try!
Update (10-22-2008): It's amazing to me that this still seems to be my most popular entry! For anyone stumbling on it after 2006,
Daemon Tools works just fine in Vista and is back to being my ISO mounter of choice!