OEM License Agreement
I've read that and there's nothing in there that says you can't transfer the license.
Here's an article with a MS rep. He states that users can transfer the license.
http://www.pcmag.com/article2/0,1759,2087792,00.asp
EDIT: Actually, I reread the article and Microsoft still claims that users can't transfer the license after it's tied to the motherboard. Well, whatever. People do anyway.