Heya, I did something similar a little while ago: http://www.flashgamer.com/2008/09/pretty_additive_cubes.html
Make sure you are using the latest version from the trunk as a bug was recently discovered with exactly this: http://groups.google.com/group/away3d-dev/browse_thread/thread/9bff1b577aeb6a52?hl=en# J
