Basically, every time you build/rebuild your .NET assembly, it needs to be in the GAC for BizTalk to be able to use it. So, I put this code on the Build Events to move the assembly to the GAC.

"C:\Program Files (x86)\Microsoft Visual Studio 8\SDK\v2.0\Bin\gacutil.exe" /i $(TargetFileName)there. Now I won't forget.