Im thing the Athlon MP is meant for SMP. Well you gotta think marketing. Why would you want XP to do SMP when you have the MP already doing it aiming at business machines.
the MP rating for the processor means its run through the appropriate tests to assure you it'll work in an MP environment. It costs more for the same chip because it costs more money to put the chips through the tests. It is possible to buy Athlon XPs, or old Tbirds and get lucky and have them run in an MP mobo just fine.