I haven't seen this mentioned anywhere, but while Parallels announced support for Vista RC1 (build 5600) for the Workstation product (runs on windows and linux), but a build that appeared on their website on Friday of Desktop (the mac version), build 1898, runs Vista RC1 quite nicely. Surely, you don't get the "cool" aero effects, and indeed you don't even get the "Almost but not quite" view. It's runs though.
I've just grabbed build 5728, and I'll give it a try.
Screenshots may appear later... maybe.