Microsoft has released a toolkit for IT departments, etc to block the update to SP2.
Here's more info:
Toolkit to Temporarily Block Delivery of Windows XP SP2 to a PC Through Automatic Updates and Windows Update
Executable to Temporarily Block Delivery of Windows XP SP2 to a PC Through Automatic Updates and Windows Update
The tool to un-block:
Executable to Un-block Delivery of Windows XP SP2 to a PC Through Automatic Updates and Windows Update
