Never mind. The AR used wasn't the up-to-date one.... I use XP 64 and my project was compiling in the \program files (x86)\pipc\PIAF\Plugins rather than \program files\pipc\PIAF\Plugins as I always use.
It should not matter whether you put the DLL in the application folder or on the GAC, just make sure you have it. On some specific DLLs having them with the app may lead to unexpected behaviour; say, for example, that you have the OSIsoft dll's lying around but have not installed PI SDK on that computer, the results are (as you may have expected) unexpected.
My advice would be to make a self installer that actually has all the files. Even if this was not the answer to your problem.