Windows Vista on my MacBook Pro (because I'm an idiot)
As many of you know, I've experienced numerous, nail-biting, revenge-worthy problems with my MacBook Pro. After many replacements and repairs, I finally just cracked and decided to wreak my vengeance in the form of callously installing Windows Vista, without regard for the Pro's feelings or my standing in His Steveness' eyes. After spending several hours troubleshooting tiny problems with the installer (ie, why it wouldn't install), I finally googled "Vista + MacBook" and came to one great blog on the MSDN network that made the whole thing very, very easy. (Install pictures here; driver updates here and here.)
Basically, David had it right: You do need to wipe the EFI partition, which can be nasty if you need to update the firmware or repartition your drive. But you don't need to have Windows XP already installed — just boot from the Vista install disk instead of the XP disk after running Boot Camp. Also, after the install, you need to Repair the Vista installation to get the thing to actually boot. The Mac Drivers for Windows XP disk doesn't work, but Vista manages to load (by default or via Windows Update) most of the drivers it needs to run. Check out the blog for more details, or feel free to ask any related questions in this post's comments — I'll do my best to provide helpful (or at least not insulting) (but no promises) answers.