Paul Thurrott has created (upgraded) is slipstream guides to include SP3.
Slipstreaming Windows XP with Service Pack 3 (SP3)
nlite claims to work with this version. For those interested they also have a method to integrate IE7, and WMP 11. I’m gonna try this in the next few days.
*Update 30APRIL08 – I’ve justed used nlite to install XP Pro, slipstreamed with SP3 and IE7. Everything went smoothly.