It seems that a very old bug (svn.open-mpi.org/trac/ompi/ticket/1982) is playing up when linking fortran code with mpicc on Mac OS X 10.6 and the Macports distribution openmpi @1.6.3_0+gcc44. I got it working by reading up on this discussion thread:
and applying the fix given there, add '-Wl,-commons,use_dylibs', to the c compiler flags solves the problem.
In addition to that discussion should be mentioned that it is in fact a link flag, not a compiler issue, so the important thing is to supply the flag to the linker, not the fortran compiler. The fix would be to add -Wl,-commons,use_dylibs to the flags supplied by mpicc, but I guess that there could be reasons for not doing that, but that is for you experts to say.
I'm posting to the users' list, because I'm unsure of whether this qualifies as a bug propre, but at least it falls
in the category "undocumented behaviour that totally baffled a fairly
experienced user used to solving his own problems and who is generally
not a whiner".