I haven't heard anything about this internally .. no press release, no all-hands mail, but according to the technical press, we're delaying XPSP3 because of a compatibility (or rather, an in-compatibility) with Microsoft Dynamics Retail Management System (RMS).
Will be well worth the wait though. XPSP3 will include about over 1,000 fixes from XPSP2 and enhancements now found only in Windows Vista.
Download sizes will vary .. a well-patched system will be about 70MB.
SysAdmins (and the idly-curious with big pipes) can download the whole 300MB package once it is available.