It's clearly Apple's aim to give Mac users a choice here. They only support the newest version of XP right now - and that makes sense. This way, they only have to test their hardware drivers against one specific version of Windows XP - and nothing older. I guess - but I guess I'm right here - they'll simply add support for _later_ Windows OSs later on. Vista was still in beta, and they probably didn't want to put you on "too beta a track" with their BootCamp beta.
I'm pretty sure we'll hear about Vista support in BootCamp tomorrow at WWDC's keynote.