It depends upon the type of license you buy, and what MS's licensing smallprint is going to say this time.
In the past it's flip-flopped between one license per PC to one license per desktop/laptop to one license per 3 machines and perm any one of three (dependent upon if you bought OEM/Open/Select/Enterprise etc).
MS licensing is one of the hardest parts of my job, and if I was Steve Ballmer i'd fire the whole marketing/licensing depts and start again.
