FastDL 2.4 was released today. FastDL provides a way to do cluster computing from within IDL in two different flavors: mpiDL and TaskDL. For problems which require communication between nodes, mpiDL provides an interface to the MPI standard. If you are familiar with MPI, then mpiDL should be very easy to use. It can now be used with MPICH, MPICH2, or OpenMPI implementations of MPI. For problems where nodes do not need to communicate with each other, TaskDL is a task farming solution for IDL.
Full disclosure: I work for Tech-X Corporation and did some work on this project.