Q:

programmer is not responding with stacked shields

I keep getting the following error when trying to upload my code with the Arduino IDE:
avrdude: stk500_recv(): programmer is not responding

I stacked the following shields:

Sparkfun ColorLCD shield (shieldlist reference)
Libelium XBee shield (shieldlist reference)

on top of a Arduino UNO.
I first thought it could be coming from the shields draining too much power from the USB port, but connecting an external power supply hasn't changed the situation (I had to set it up to 7,5V/600mA to avoid overheating).
I'm kind of lost here, judging from shieldlist I should only be worried about the power supply, which is obviously not the case.
Any idea about my problem?

A:

The culprit will be the XBee shield.
It uses the TX and RX pins.  These are also used by the programming system.  If anything is connected that uses those pins while you try to program it can interfere with the programming data.
I usually design shields that use the UART to include a pair of diodes to isolate them from the rest of the UART system and allow programming to work while the shield is connected.

