By combining threads and a remote procedure call (RPC) package, you can exploit nonshared-memory multiprocessors, such as a collection of workstations. This combination distributes your application relatively easily and treats the collection of workstations as a multiprocessor.
For example, one thread might create additional threads. Each of these children could then place a remote procedure call, invoking a procedure on another workstation. Although the original thread has merely created threads that are now running in parallel, this parallelism involves other computers.
The Message Processing Interface (MPI) might be a more effective approach to achieve multithreading in applications that run across distributed systems. See http://www-unix.mcs.anl.gov/mpi/ for more information about MPI.
The Sun HPC ClusterToolsTM software includes Open MPI Message Passing Interface (OMPI), which is an open source implementation of MPI. See the Sun HPC ClusterTools product page for more information about ClusterTools.