I see that you used an XBee module. Can't you downsize the whole thing by using another chip with a zigbee stack like a CC430 ?http://focus.ti.com/docs/prod/folders/print/cc2430.html
I have a question about working with the XBee, can you use the arduino with the Xbee shield to give a PC a Xbee device? I am working on a device powered by the Arduino, and I need Xbee as part of the application, and I want to know if I can use the USB with the Arduino to control a set of relays and as a Xbee device at the same time
Did you consider including (or did you) a six-pin header (possibly unmounted) to allow the board to be programmed using an FTDI cable? That would lower the cost needed to start loading sketches onto the board.