Anyway, i had an issue with the Killer.. Basically, after a minute or so of operating, the arduino board freezed and the robot became unresponsitive.
After some heavy debuggin, i didn't found anything. I've noticed that using standard Arduino Serial library instead of AFSoftwareSerial that i was using, it works. I used that to communicate with the XBee module. So it must be something with AFSoftwareSerial, that btw, if you don't know, is a library for serial communication in software made by LadyAda. The serial connection is heavily sinchronized, so making it in software is an hard job. Maybe something in my setup messes with it, and so it hangs. Switching to the standard Serial, which works in hardware, fixed the problem.
The only problem is that the primary serial port is also used to download programs into the arduino, so i'll probably have to disconnect the xbee every time.. Well, whatever :)