One important issue for developers is how the windows XP service pack 2 affects software we write today. You'd be surprised but there are some “breaking” changes inside it, stuff we should be aware of.
Today this turned up in the MSFT Downloads RSS feed:
Changes to Functionality in Microsoft Windows XP Service Pack 2
This document focuses on the changes in Windows XP Service Pack 2 and its implications for developers. Examples and details are provided for several of the technologies that are experiencing the biggest changes. Future versions of this document will cover all new and changed technologies.
from a quick browse I can tell you it's a good idea to have a browse through and make sure your code won't break with the new service pack.