I have a Zenith 50" plasma that is made by LG (I don't remember the model number off hand) that has RS-232. I have it connected up the to the HA server and can control all the functions nicely.
Eric
Did you write the control yourself, or did it come with a HA package like ML or CQC?
I use HouseBot and it has a serial device that you can easily add commands for it to send to the TV.
Eric