LG is about the only manufacturer that consistently puts RS-232 control into their units, but even then they don't do it for every unit. If you are looking at one in the store, flip around to the back and look for an RS-232 serial input. If there is one, you want to make sure it is labeled "Service and Control" - if it only says "Service" it won't work. If you are looking on-line then you can check the specs for "RS-232 Control".
LG also makes some other brand panels like Sharp and Zenith, so you can find RS-232 control on some of their models too (but not as many).
I realize this isn't the same as IP control, but there are a lot more panels that have RS-232 control and if you really need IP control you can always get a serial to IP converter box.