A few of the responses are a little OTT for my liking. OEM allows you to swap everything else except the motherboard right? And thats ok? But if you want to keep everything else and swap the motherboard you're scum? Bit much?
Actually I imagine that MS would still say you were fully licensed if all you did was swap the motherboard and kept everything else. Even if they didn't it isn't like they are going to take you to court over it. Not to mention the terrible PR it would cause, I sincerely doubt any judge would enforce the EULA if all you did is change the motherboard. Yes, the EULA is a contract. And as such it has to be a fair contract. Doesn't seem fair to me if someone can get away with swapping everything else on their computer but you are in the dock for only swapping one part.
Now I, and probably Microsoft, draw the line if you swapped every other part AND the motherboard on an OEM license. I believe the clause in the EULA is stop people transferring the license from computer to computer and they picked the motherboard as the most likely place to tie the license to. In my opinion if all you are doing is changing the motherboard and leaving everything else it isn't a new computer.
A little common sense can go a long way.