I am using Some arduinos (5V Pro-Mini, but planing to move to actual ATMEGA chip) over a distance of few metres.
These are connected using standard Alarm cable (I had problems with Ethernet cable and 12 volts leaking between cores) In mine set-up the cabling caries DC~12V (bit bigger closer to the source and backup car battery)I am protecting mine inputs using resistive divider (all the inputs where designed to be around 12VDC(then dropped to around 4.3VDC when HIGH) ground is connected through diode to protect polarity.
Problem is How to protect the RS-232 from being either shorted to or connected to 12DC (or ground) incorrectly. I am planning the possibility when cabling may extend up-to 60-70 metres.
I have been considering the Zener diodes, but I am not sure how effective would 5.1V + margin be. while the 4.7 model might be too low and short the standard communication Although I do not know what level the internal protection has from over-curent against ground.
Edit: I am using UART on a chip (directly on a logic level) the baud rate is intentionally slow and data send (including self-diagnostic for data corruption)is minimal generally less then 2 bytes per second.