So more learning, never mind the day time ...
After rummaging around, the problem is not that it cant find the libmpinet library but that its is the wrong type, apparently
c101078:PingPong stuart$ MONO_LOG_LEVEL=debug mono PingPong.exe
Mono-INFO: DllImport error loading library 'dlopen(./lib/usr/local/lib/libmpinet.0.dylib.bundle, 9): image not found'.
Mono-INFO: DllImport loading: '/usr/local/lib/libmpinet.0.dylib'.
Mono-INFO: DllImport error loading library 'dlopen(/usr/local/lib/libmpinet.0.dylib, 9): no suitable image found. Did find:
/usr/local/lib/libmpinet.0.dylib: mach-o, but wrong architecture'.
And these appear to be mach-o 64 bit libraries
c101078:MPI stuart$ file /usr/local/lib/libmpich.dylib
/usr/local/lib/libmpich.dylib: Mach-O 64-bit dynamically linked shared library x86_64
c101078:MPI stuart$ file /usr/local/lib/libmpinet.dylib
/usr/local/lib/libmpinet.dylib: Mach-O 64-bit dynamically linked shared library x86_64
>From my reading Mono on OSX is only 32bit, should these be libraries be 32 bit or can MPI.Net be built to call the 64 bit versions (I have read about other examples of this being the case, mod_mono)?
Is this a Snow Leopard 64bit problem?
Otherwise does anyone know how to build MPICH2 with both 32&64bit combined libraries (ok, probably for another list).