These units look great! I built a similar thing with 2x2 units but using sparkfun's serial matrix... (LED Box running Conway's 'Game of Life' - YouTube)
One question - is there / will there be a version with a diffuser on the front to help the colors mix? One thing I like about the sparkfun matrix is each pixel has an opaque cover. From the pics it looks like the rainbowduino's RGB led chips aren't covered.
Cheers
Nick