1) Any "OEM" copy that you can buy from a store is system builders (as contrasted with the royalty oem copies that come PREINSTALLED when you buy a branded computer).
2) A system builder copy was originally meant to be sold to white box vendors who would then install it on a computer meant for resale to a 3rd party. So if you bought a PC from a mom and pop store, thats what you would find. The EULA was abused later by coupling the license to an insignificant piece of hardware such as a mouse.
In Windows 7 at least, system builder copies are freely available from respected stores like newegg and tigerdirect which leads to the conclusion that MS is OK with it.
3) System builder is cheaper than retail copies because legally they are tied to the first mobo they are installed on, they are NOT transferrable. Second, MS support is not bundled with them.