From Ocean's Control, the manufacturer of the relayduino, I received the following tips:
You would have to make sure all the connections from the ICSP header that are required for the xbee shield are connected to the right pins I haven't used it with an xbee shield for quite some time and I don't know if the xbee shield has changed at all.
You would need to make sure that it is also set up correctly with regard to the pins that are not used for communication.
If they are connected on the xbee shield and set as an input or output and the 223 has something different configured that could cause a problem.
Is he loading the same sketch to the uno and the 223?
Are the baud rates correct?
There are so many possibilities, it will most likely be something wrong with wiring or configuration.
Although it didn't help me I thought I post it here for others that might land on this page trough google.
I tried to setup pin 0 and 1 to accomodate the TX and RX:
void setup() {
Serial.begin(19200); //xbee chips are configured at 19200
Serial.print("start xbee");
pinMode(0, INPUT); //RX
pinMode(1, OUTPUT); //TX
}
void loop() {
Serial.print("test xbee");
delay(1000);
}
But that doesn't help, and al the others pins are not used by the Xbee but are just passed trough. http://www.arduino.cc/en/uploads/Main/XbeeShieldSchematic.pdf
The only pins that are used by the ISCP are the grnd, 5V and the reset line (although I hardly believe leaving the reset line unconnected can obstruct the shield from working)
If anyone has some more ideas to debug this problem I would be happy!