Nokia doesn't have loyalties.
They're company and the openly admitted prime directive or any company is make money.
If they want W7, they get W7. I see no problem here. Not in implementation (hardware is pretty standard),
not in conflict of interests (multiple OSs run now just fine, one more won't matter).
Don't know if it will happen or not, but I certainly see little problem. In fact, since Windows has little allegiance to one line of products (there's a truck full of stuff running Win)
I can't think of an instance where having the option to switch from Linux to Windows would hurt sales.
Frankly, I'm not entirely sure why N900 doesn't run a CE as an option already.