Legally, you can't use an OEM license on any piece of hardware that it didn't come on originally. IE: you can't use that license on your MB or anything else for that matter once you upgrade / install 7. It's just meant to be a dead sticker.
Even then, many folks have transferred OEM licenses simply by calling Microsoft and asking.
Note that the EULA is a legal agreement between you and Microsoft, not the law of the land. And Microsoft are perfectly capable of going above and beyond the specific rights explicitly granted to you in the EULA if they darn well feel like it that day.
So far, all retail upgrade EULAs I have ever read DO explicitly grant you transfer rights, which is why I was suggesting this as a possible way to go about it perfectly within the EULA.
Situation: Vista OEM on old machine not transferrable, new retail Windows 7 is.
Solution: Upgrade the Vista OEM box to Windows 7 using a retail upgrade, then exercise your transfer rights and move it over to the Mac.
You can usually skip the step of actually installing it over Vista on the old PC, and go straight to installing on the Mac. Just remember to decommission the PC. If you want to maintain some continuity of hardware stick the HDD from the PC in an external enclosure and use it with your Mac.
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