This weekend we posted RC 2 of the Windows XP SP2 Platform SDK. You can use this to get the headers and other files you need to do XPSP2 related development until the service pack is released.
Microsoft Windows XP Service Pack 2 Platform SDK RC2
This is the Platform SDK for Windows XP Service Pack 2 Release Candidate 2. Developers can use this to build and test pre-release applications for Windows XP Service pack 2.