Note that rsh != ssh. It;s a different (older and less secure) protocol that's still used quite a bit in high performance computing applications (as it does not have the encryption overhead of ssh). The rsh protocol is not enabled by default on Fedora, you'll need to edit the files in /etc/xinetd.d and enable it. I usually enable rsh, rlogin, and rexec in HPC encironments. You will need to restart xinetd after enablingthese services.
Note that if you're using LAM-MPI that is has been surperceded by the OpenMPI project. OpenMPI supports job launching over SSH as well as RSH. I'm not sure if LAM-MPI ever supported this as I used MPICH before switching to OpenMPI.