OK, this is fun. I booted my computer into Windows this morning. But I'm posting this from Linux! (Xubuntu, specifically.)
VirtualBox is free virtualization software from Sun- basically, you can run a PC-inside-a-PC, booting whatever OS you want on the inner one. It's not an emulator- it runs as much code as possible directly on the processor, recompiling some portions as needed.
So you can run Linux on Windows, Windows on Linux, Windows XP on Vista, any of them on Mac...
Did I mention it's free? And surprisingly simple to use. Just setup the virtual hardware and boot.
You do need to get a CD to boot off of at least the first time (until you can install to a virtual hard disk); real CDs and downloaded images both work. Also, you need enough memory to run both OSes, which is why I'm running Xubuntu. My new computer, when it arrives, should handle the full Ubuntu.