If you are involved in writing Windows apps that are network enabled, particularly if they accept incoming connections, you should take a look at this:

http://www.neowin.net/comments.php?id=15873&category=main

Quite a few apps are going to get broken I think. The biggest change is that if an app wants to listen on a port it now has to register with the WinXP firewall and can only do it when running as an administrator.