A few bits:
What's your budget? If you have, say... $300, you can be up and running early next week. If you want to do it for $100, it's going to take a bit more work, but be easier. If you need to do this for $50, you're in trouble.
What's your timeline? If you have the time to learn more about electronics (the software for this is reasonably easy), you can construct such things as a DMX shield, then put an RJ-45 jack on it, as you seem to suggest that you're locked into cat-5, or just prefer it.
If time is short, you can LIKELY (depending on environmental variables) go with XBEEs to communicate between the units. 40 meters is not a problem, the noisy environment of a theater can kill reception, though. Noisy lights, lots of steel.... Blech. Still, though, I'm 99% sure you could do it with easily returnable parts if not.
Is there a LAN in the theater with drops at the locations you need to hit? Could use actual TCP/IP to do it.
Cheap and fast? voltage converters made with spare MOSFETS that would take your serial connection and bidirectionally convert it to logic level at each unit. Throw each unit in a Radio Shack project box and call it done. With the application being so simple, you could probably even use simple transistors to dump a 5v 500mA signal down a line of the cable that would trip a transistor on the oth..... I'm getting silly.
OH WAIT: 3 watt IR LEDS. You could also (~$12 in parts for just the comms) build an IR lLED based comm system using psycho high powered IR LEDS.
Remember that if you are AT ALL unsure of your comm system, each unit should be sending it's status frequently. There's no reason not to.