The post that merits the attention states in part:
My new Asus A266M-D won't work in dual mode with AMD-XP's - at boot it reports:Needless to say this is an interesting development if true. Independent confirmation on this would be nice, but 760MPX boards are more or less impossible to find at the moment, so that will probably have to wait a while.
"AMD XP CPU detected - forcing single CPU mode"
According to Asus this is a requirement being placed on manufactures of 760MPX boards by AMD.
If it is true, the next question to ponder is how is the 760MPX chipset determining the difference between an Athlon XP and an Athlon MP? My bet at this point is on the L1 bridges; when I get a chance, I'm going to spend some time looking at the bridges on an XP and an MP side by side.