"...AFAIK, it's Windows policy that the retail Windows license applies to one machine only. you get to pick whether that's a hardware (BootCamp) or software (Fusion) machine. if it's an OEM license, the license is tied forever to the hardware it was first installed on. any other usage is in violation of the license..."
Actually, "retail Windows license" is an unclear phrase, since it could refer to either the "Builder Edition" (OEM) or the more expensive "full" edition. They've changed the legal language with Windows 7 and it's even tougher (narrower) now than it used to be. The Builder (OEM) Edition is tied to one machine, as you say, but even then that's only allowed if it's a machine you build for resale -- in other words, you're a professional machine builder. Otherwise you're supposed to get the full edition, which is licensed to the individual, not to a machine, and can be transferred from one machine to another as long as it's for the same user and they only use one machine at a time. At least, that's what most of the recent online descriptions of the license say; I'm not a lawyer or expert in this.