I am considering using the XBee ZigBee-based modules – I have a couple ordered and on the way, but yes, I think I understand what you are saying about the Tx modules being unable to differentiate one from the other. That explains the crazy malfunctions the moment I pop another Tx module in.
When the XBee modules come in I'll hook them up (based on the Arduino Playground) and see if I can get this to work. The Arduino BT is out for me, as it is too expensive, and also I am making small-sized arduinos (they need to fit into containers slightly larger than the size of a pepper shaker). The mini's an option, but it's still cheaper to go the DIY way for now.
I'll keep this post updated as I go along.
Thanks for all of your feedback!