I use my arduino as driver for my rainbowduinos (led matrix driver) - aka serial-to-i2c gateway. The serial communication was kinda tricky but work very stable now. with the processing library i can now send frames from my pc to one or multiple led matrix displays.
blog entry: Rainbowduino fun – aka neorainbowduino
source repo: http://code.google.com/p/neorainbowduino/