Rmpi, mpich2 and Red Hat Setup Question

We are trying to use papply and Rmpi on a linux computer (RHEL v5.2)  
where Rmpi was built with mpich2 v1.1. We have read the rpmi  
documentation but can't figure out how to tell rmpi how many processes  
we want to use.

We issued an mpdboot command where we specified the number of proceses  
to use in a hosts file.  How do we tell R how many processes to use?  
We are running R v2.9.1.

So far we have tried the simple examples from the papply help but they  
indicate they are running serially.

Thanks,

Tom