With the XP license, you were required to buy a piece of hardware (people on Ebay used to ship cables) to qualify for a purchase of OEM software. With Vista the requirement was removed, and in the past the idea of a system builder was anyone who built their own pc essentially.
The current rules mention nothing about a requirement to purchase hardware. A system builder (who you bought it from) can distribute the license as they see fit. It is not their requirement to ensure that the pc you install it on to is then on-sold. The link posted by sidefx should update everyone
Essentially, they are never going to police an individual who has bought an OEM version for their own personal use. Plus there are too many examples where MS employees have said it was ok to do so.