Well that didn't take very long - here's our authoritative response as described by Scott Field - one of my favorite architects (super smart, genuinely nice guy, very helpful):
http://blogs.msdn.com/windowsvistasecurity/archive/2007/08/03/x64-driver-signing-update.aspx

He describes our 3-pronged response at the end . . .
It's an interesting game that we've decided to play here (although it is one I firmly believe we must).
IMHO it will be interesting to see if the kernel mode revocation mechanism is the next thing that will be attacked here . . . I'm sure it will be.  We live in interesting times. :)