There’s a cluster function in the raster package!
Increasing the speed of {raster} processing with R: Part 2/3 Parallelisation
Increasing the speed of {raster} processing with R: Part 3/3 Cluster
http://r-sig-geo.2731867.n2.nabble.com/Re-Parallel-predict-now-in-spatial-tools-td7586048.html
http://www.timassal.com/?p=1920