I agree that Microsoft has nothing to lose and a lot to gain from allowing Macs to run Vista. We've had this discussion a million times that MS is a software company. As long as they get the sale, they could pretty much care less if you installed windows on a toilet. So I doubt that they'll prevent Vista from installing under Bootcamp at least. Virtualization is a different story as has been mentioned, as virtualization might allow for some back door access to the software.
The other thing is reinstallations of Vista. I don't think that will be much of a problem (taking one of the 2 machines). It'll be tied to the same hardware as it was before, and should recognize it as the same machine. Of course, there's the issue of hardware upgrades, but this was essentially the same thing that we saw in XP. You were able to activate XP on a fixed number of machines, and certain hardware changes (CPU & Mobo) forced the user to reactivate Windows, while things like GPU, RAM and HD upgrades didn't.