XP powered Eee PC: $400, and April 9th. The hardware's identical to current-gen Eee PCs, though, so if you're happy with your little white lappy and are itching for XP you might try to pick up a license on eBay. Still, if all goes well for ASUS, they'll be selling 4x what they did the first time around, innit?
P.S. -If you are, for whatever reason, itching to read more about current hardware running the 7 year old OS, Laptop's got a hands-on. Nothing that wasn't already out there though.