Xbee shield with seeduino mega trouble

Hi everyone,
I'm experiencing a very odd problem. I've been testing Xbee Series 2 radios for my installation. Everything works perfectly fine when I use a Seeduino Mega board with an regulated xbee explorer breakout board from sparkfun , or when I use a regular Arduino Duemilanove with the Xbee shield. However, for some strange reason, when I hook up the Xbee shield to Seeduino Mega, no data gets received on my PC, while I can still send data to the seeduino. as soon as I swap the shield for the sparkfun board, everything starts to fly again. I've tried different boards and shields, always the same problem. Radios work perfectly fine when tested on two terminals. Any ideas what could be the cause?