*sigh*. It seems you can't strong name a C++/CLI assembly, i.e., sign a managed C++ assembly, with a .pfx file like you can in C# or VB in Visual Studio 2005. You have to revert to using an unprotected .snk file ;(
An MSDN forum post about this issue was never answered. It seems that the Visual C++ team is even unaware of the possibility of using a .pfx file to sign a managed assembly in their sister products. And the writers of this MSDN article were apparently unaware that Visual C++ is also a part of Visual Studio 2005 since they assume it can be done using both a .snk and .pfx file.