The MPI specification predated Beowulf clusters, my friend. Before this time, you had computers of varying numbers of CPUs. It was conceived that you might have clusters of computers, but nothing on today's scale. Besides, the guys who dreamt up MPI were computer scientists, ie, not hardware guys or systems guys. MPI-2, which has the ability to spawn and connect to separate MPI instances, doesn't make this easy.
Search for MPI-2 libraries that support process/communication attachment/detachment. You might find something there. Please post back if you do.
UPDATE: See this PDF/slide presentation
http://www.cs.utk.edu/~dongarra/WEB-...2-features.pdf
Search for "Process Management". You use "MPI_COMM_SPAWN" to create a new set of processes with the same arguments on the command line, but now you must use an "INTERcommunicator" (instead of INTRA); you can do MPI_SEND/MPI_RECV, but not collective functions. Still, if a node dies, this doesn't help!! You would basically need to create your own process and communication management on top of MPI. That's why I suggest you look for a library.