If you had any issues with .Net FX 3.5 SP1, or if you are holding it back for some reason, it’s time to head over to KB959209 and get this patch.
It contains a set of fixed for both the .Net 2.0 and the .Net 3.0 bits that were patched as a part of this release. There is also a list of fixes incase you have been noticing some strange behavior lately.
The .Net Fx 3.5 Service Pack 1 contained quite a few interesting optimizations, and is well worth the install.