j3pflynn, by XP-M I think you mean mobile XP (vs. the multiprocessor Athlon MP). The mobile XP uses the same die as the desktop XP, packaged for implementing in laptops. Run at a lower clock rate, it uses less power than the desktop versions, but still more than what one wants for the slim & light crowd. AMD really hasn't focussed on this segment. They are leveraging their desktop design for the more powerful laptops and focussing their design resources on Hammer.
The Athlon4 is an older processor, Palamino-based I recall. Here are the mobile offerings: