For future reference, there was no problem. A copy of everything to/from the XBEE is sent to the serial monitor anyway, so I could program as though sending to the XBEE also sent to the PC and had the transmitter ignore the reciever's messages.
Now I have another problem (probably shouldn't start a new topic for this).
How do I access interrupts on the arduino UNO? I'm receiving 40kHz pulses that last 4us each. Can interrupts trigger off this? If not then I'll have to put an integrator in the circuit to lengthen the pulses.
Less important, I'd like to replace the polling code with a proper hardware timer. The docs are a little fuzzy on how to set this up.
int i = 0;
while (digitalRead(ultrasound) == HIGH) {
i++;
if (i == 0) {
break;
}
}