NGOHQ writes AMD seems to have stopped monthly driver support for Windows XP. The last Windows XP driver available on AMD's website is Catalyst 12.9 beta, the WHQL version isn't available for Windows XP and the newly released Catalyst 12.10 WHQL nor Catalyst 12.11 beta aren't available for Windows XP either.
This could be a strong indication that AMD is planning to discontinue monthly driver support for Windows XP. This won't be a surprise as some hot upcoming PC games (Black Ops II, Hitman Absolution) and even software (Photoshop CS7) won't support Windows XP either.