At the moment, ExternalLibraries/MPI guesses that the library list for openmpi is just 'mpi mpi_cxx'. This is wrong in my case (even for the built library; I need additional libraries. The OpenMPI FAQ mentions this: "NOTE: It is absolutely not sufficient to simply add "-lmpi" to your link line and assume that you will obtain a valid Open MPI executable."
They do recommend to use the compiler wrappers. I didn't try that, leaving a patch to a minimum. I would also not be sure how to replace to actual compilers using a thorn while it is supposedly being compiled - after the Cactus configure state (cannot do it before since the wrappers might not be build yet). Thus, I go the second way - also shown in the FAQ: I use mpic++ to get hold of the flags that are needed when linking against the library. This is very similar to what happens for some of the other ExternalLibraries as well (but a bit simpler since they also provide options to just get a list of the libraries, not including the flags themselves - which is what Cactus expects).
In my case, this uses the libraries 'mpi_cxx mpi nsl util m m nsl util m dl openmp'. Especially the missing 'util' in the current list prevented me from linking. Adding it hard to the list however might be wrong on systems where this is not needed or that library might not even exist. (and this isn't the only missing library)
I would like this to be included in the next release, as this would otherwise prevent people from building on at least some of the major Linux distros out of the box (Debian wheezy here) - or we would have to specify the library list by hand for these systems. Given the central part MPI plays I set this to 'major'. If we see problems with this approach we can still guess the missing libraries and try that, until after the release.
OpenMPI FAQ: http://www.open-mpi.org/faq/?category=mpi-apps