I also have a LED Display and would love to control it from my PDA I have a Zaurus, but I am interested in all projects like this as I only just took delivery of my LED display.
I too have a Zaurus PDA and I use it to control all sorts of things (motors, lights, relays, LEDs) and to measure things (light, switches, etc.) via our product and the serial port. Feel free to contact me directly.