I've documented Arduino serial ports here:
Gammon Forum : Electronics : Microprocessors : Async (Serial) peripheral interface - for Arduino
Wow, thanks a lot. That's exactly what I was looking for & more.
I should probably say that I need the "regular" serial port for diagnostics, and later it'll most likely be connected to a LCD. I need a single communications port to let the Arduinos talk to each other, and I'm using the SoftSerial library for that. It's possible that the other Arduino might not be on sometimes, and I don't want it powering up through the pins - if inline resistors won't work, is there some way I can isolate the pins so that I won't blow up any more Arduinos?