I think the bigger issue is new voltage requirements. The thoroughbred will be on a .13 process, which will demand much less voltage. So, it will use the same socket, but not all motherboards will have voltage regulators that will be able to supply the correct voltage. It is possible however that you could still run a thoroughbred in such a motherboard, but with a higher voltage- this would probably be bad for it. In short, some current mobos will work, but alot wont.
As for the increased FSB, i dont think it will happen:
1. DDR333 is too expensive/ has not been officially adopted
2. The athlon doesn't see too much performance increase from a higher fsb (not like northwood P4s anyway)
3. AMD will be focusing on the new "hammer" line of chips, the thoroughbread is simply an Athlon XP with a die shrink to allow for hihger clock speeds in the interim until the hammer chips are released
Read this somewhere recently, cant remember where (toms hardware??)
Butch