Xgrid isn't what you'd be using with Compressor or the rest of the FCP Studio suite of applications. Applications like Shake (compositing) and FCP Studio use something called Qmaster but only for things like batch processing and stuff.
Qmaster was introduced with Shake and is the tool that allows Compressor and DVD Studio Pro to combine the processing power of up to 10 Mac or Xserve systems to compress video files. Using Qmaster over a standard NFS network, jobs can be segmented among computers to speed compression, which means computers that arent busy can offload tasks from computers that are. This capability can vastly speed the process of compressing files into MPEG-2 for DVD Studio Pro. Included with Qmaster are QuickCluster, which simplifies getting started, and Qadministrator, which provides fine-grained control for more complex setups.