I saw a question posted on the GloblBank wiki about why developer's aren't participatng on the GotDotNet message boards.
I don't claim to know the answer for all developers but personally I participate in SourceForge instead of GotDotNet. And indeed there are a large number of .Net based projects in SourceForge so perhaps I am not the only one.
The reasons I use for SourceForge instead of GotDotNet:
1. In SourceForge I can always read the code. GotDotNet I may or may not be able to. This has nothing to do with any philosophy towards open source it's just that as a developer it's often what I am interested in as oppossed to just getting some binaries.
2. There is a lot more activity in SouceForge. This is a chicken-and-egg argument.
3. GotDotNet seems in some ways like a Visual Source Safe for the Internet. My team uses VSS to store our code. While I would feel comfortable hosting our code in SourceForge (if it was open-source) I wouldn't feel so comfortable about GotDotNet. This is mainly due to the maturity, support arrangements and access controls in the GotDotNet system. As it becomes more stable perhaps this will change.