hi
if you poke around the forum you will find lots of posts on this. I know that several people have done it, using various techniques: RS232, RS-422 etc. For example there is an exhibition in Montreal right now by someone that uses 40 Modern Device "Barebones" Arduino-compatibles networked together, but they haven't postd how they did it yet.
Use Google to search the forum, as the forum search only goes back a few weeks for some reason.
One problem you will encounter is that during boot up the serial port(s) are used by the Arduino to check with the IDE.
D