Hey! Norwegian university student, just ordered an Arduino Diecimila.
1st question, how to make two Arduinorobots communicate and act according to the info the other outputs. ?
How to expand digital\analog I/Os ? [do I need another microprocessor, is there any limit in I/Os?]
Excause my n00b....ness? 
Are the robots going to be moving around, then i think the Blue tooth Arduino board might be a good solution.
For expanding Digital or analog In / out puts see here:
look under M for Multiplexing, there you will find links to a number of articles about expanding the number of analog and digital input and outputs.
A alternative to bluetooth is wire up a infrared receiver and led.
You'll have to write the protocol yourself however.