If you’ve been using the Facebook Developer Toolkit in the past to develop ASP.NET iFrame applications for Facebook, you may be interested in a few updates. That SDK has not been updated since April of 2010. And Facebook has made quite a few changes to their API since then. So I'd consider this project all but abandoned, perhaps resulting in things you expect to work not working.
However, you may want to look into this Facebook C# SDK, which looks more promising (with the last stable build coming at the end of October 2010). And if that didn’t brighten your mood enough, how about the fact that this SDK has also been added to NuGet!