If you’re developing BizTalk 2009 solutions (Orchestrations) and you’ve split your schemas out into alternative assemblies (projects) – sometimes you’ll get odd not found issues with some (if not all) of the types in those referenced assemblies.  You can try everything – recompile, de-gac, re-gac, – doesn’t matter.

Well there’s a hotfix for this:

http://support.microsoft.com/kb/977428/en-us

FIX: You experience various problems when you develop a BizTalk project that references another BizTalk project in Visual Studio on a computer that is running BizTalk Server 2009