Microsoft has produced a whitepaper on the forthcoming XP Service Pack 3 here. According to the Microsoft website:

Windows® XP Service Pack 3 (SP3) includes all previously released updates for the operating system. This update also includes a small number of new functionalities, which do not significantly change customers’ experience with the operating system. This white paper summarizes what is new in Windows XP SP3.

While there isn’t a lot of detail, the whitepaper does provide a look at what changes users should expect.