The folks at the OSx86 project seem to have Windows Vista up and running on an Intel based iMac! I noticed in the post that they don't have Aero running, and that they used the ATI drivers from Apple's site.

ATI's latest Windows Vista drivers might work better than the stock drivers from Apple which are designed for Windows XP. I haven't tried this myself (I don't have an Intel Mac) but if it works for anyone out there I'd love to know about it!