My computers that I have joined using this process:
Countless Windows 2000 Workstation
Countless Windows XP Professional
SuSE Enterprise 8 via Samba 2.2
Mac OS X 10.3 via Samba 3.0

So whatever is going on behind the scenes, it is working for XP as well. This includes a base XP install off a normal CD, thus nothing weird in policy manager is being changed client side to allow them to join.