If you ever deploy a .net class library that isn’t being called by a .net application and references items in the config file then you will have to determine where the .net configuration subsystem will look for the config file and what it should be called. In hindsight, the answer seems rather obvious but I spent several hours figuring this out one day. In my case, I was deploying a managed DLL with a thin COM wrapper that would be called by Excel. In order to get this to work, we placed all the DLLs in the same directory as Excel.exe, called REGASM on the generated .tlb file, and named our config file Excel.exe.config.
Like I said, so obvious in hindsight but not the first (or tenth) thing you think of.