Using a couple of 'duinos, I've demonstrated that it IS possible to have multiple Arduinos in a mix of Masters and Slaves on a single I2C bus. I've been able to use different Master/Slave addressing (IDing both), send and receive not only data, but text, too.
I based my code on a pair of sketches I found here http://absences.sofianaudry.com/en/node/10, and built on it. You can download a zip file with my finished programs (one each of Master and Slave) from my website: http://sites.google.com/site/adifferentlemming/Home/projects/arduino. My sketches are pretty well commented, so anyone wanting to get multiple Arduinos communicating shouldn't have any problems. 
I wasn't sure where else to put this post; if an Admin type wants to move it, that's fine -- maybe we need a separate "How To" on the forums for stuff like this?