I'd say if anything servers will be running 128bit a lot longer before home based computers will be just due to the memory capacity but i would say it will have backwards compatibility, as if anything m$ have learned that lesson the hard way and they could possibly have a xp/vista/win7 mode as an additional vm addon like win7 has for the XP Mode.
The way memory is updating it wouldn't surprize me if were a couple of versions of DDR up by that stage, i was actually thinking about switching from win7 32bit to 64bit with 12gb DDR3, but might wait for the new year before i take that step.