I recently used the GParted partition editor to split and dual boot my Vista laptop. This (well, at least the version I used) is a Live Linux CD - you put it into your CD drive, reboot your machine, and run the editor. It then does its thing, and voila - your partitions are changed! It was a fairly easy process, but, as all these type of activities are, a bit nerve racking. When you're playing around with that much data, you wonder if it's going to come back alive. It did, though!
I haven't put Linux on my laptop yet, but I'm considering it. Hopefully the partition editor will work its magic again without any issues.
Before you use, though, backup, backup, backup!