RegFree COM working from C#, NOT working from VBA
Using Process Monitor, I discovered that the dll was being searched for in C:/Program Files/Microsoft Office/Office11/. I moved all my dlls over into there and the error was gone.
On a sidenote, if anybody knows how to tell Excel not to look there for my files but instead where the .tlb was found, I'm interested. (I already tried adding the path to the PATH environment variable)
I can't say I've ever tried to use that technique and I've done a lot of installs over the years. I have to wonder, why not just register your .NET COM Visible server? Regasm tends to create very clean and light weight COM registration in the registry. Far cleaner then the old unmanaged DllRegisterServer() stuff.