For people who do want to migrate to XP X64 when it ships, Microsoft plans to offer a Technology Exchange Program that will let customers who bought 64-bit PCs with Windows XP Pro swap that OS for XP X64. Details are still in flux, but Microsoft's Marr says the company intends to supply new Product Keys with the new OS and then deactivate the 32-bit Product Keys.
Note, though, that you will not be able to upgrade an XP Pro PC to XP X64: You can acquire XP X64 only as a clean install, which entails backing up your existing data, installing the OS from scratch, and then reinstalling your apps