Hi Todd,
Two things:
1. I do not know how the solution conversion works, but to compile the GRC files, you need a complex custom build step. Please check the custom build step attached to the GRC files in VS .NET 2003, or look inside the .vcproj files you're trying to convert. Make sure the build step stays the same in the converted VS .NET 2005 project as well.
2. As ArchiCAD was compiled with VS .NET 2003, it's usually a bad idea to compile your Add-Ons with another version. Even though the function addresses (and decorated names) might stay the same, it still means you're using a different runtime library in your Add-On than ArchiCAD does. Which means that if in a given function ArchiCAD allocates memory in one runtime library, then you throw it away with an other one... Well, who knows what kind of errors might pop up? Simply put, Add-On development is not supported this way.
HTH:
Andras.
PS: I do not think you two have the same problem though...