Yes, our client-side NAP platform will be available on XP. In fact, my non-Vista laptop is running it right now. :->
I can't discuss release dates and such, but know that it is coming (and already working).
Jeff Sigman [MSFT]
NAP Release Manager
* Remove the "online" to actually email me.
** This posting is provided "AS IS" with no warranties, and confers no rights.