Read this for an in-depth explanation of member function pointers:
http://www.codeproject.com/KB/cpp/FastDelegate.aspx
Gist is that when you get a member function pointer you have a complex, compiler-dependent structure -- the language just doesn't guarantee that you can deref it back to an address.
SourceHook can do some decoding for you - probably only virtual calls at the moment though. See SourceHook::GetFuncInfo (and then you can use standard vtable inspection (or SH_GET_ORIG_VFNPTR_ENTRY in MM:S 1.6+ API).
Otherwise, see Don Clugston's excellent article for the structures each compiler uses.