The bug will not be present in the real RC1 build. That is, the driver will NOT be blocked in Vista RC1, and avast WILL work. I have this confirmed from MS (I had a phonecall with responsible people yesterday).
As a matter of fact, they were very polite, probably realizing that if this happened with a RELEASE build of Windows, we could actually SUE them for this!
Greetings from Madeira,
Vlk