We have been trying to track down a problem with our blog aggregator software B.AG which is currently powering INDA's blog aggregator at www.developers.ie/blogs.
We recently upgraded the server to Windows Server 2003 SP1 and for some reason, threads were simply disappearing from our custom threadpool and the application logging which uses log4net would simply stop without warning. We have just tracked the problem down to the System.Threading.Timer class which according to this has major issues when running on Windows Server 2003 with SP1. It seems the Timer class stops firing at some random point and never fires again...
I know of a number of major ASP.NET sites around the world that have applications that depend on functionality provided by the Timer class and I'm very surprised that this issue has not seen more publicity. So this is just a warning to ensure that you don't fall victim to this issue!
As far am I am aware there is no hotfix available for this problem yet.