I'm having the same issue with Intel MPI 5.1.3. "mpirun -r ssh" works (or --rsh=ssh), while setting I_MPI_MPD_RSH=ssh doesn't help. To my surprise, -r or --rsh is not documented in mpirun help. Rafal, thank you.
Dmitry, pointing rsh to ssh requires root privilege, and will affect all users. How can this be the easiest way? Thanks.