I built this interface board to allow faster, easier communication to a PC from a Jennic Zigbee module. The FT245BM USB interface chip is wired up in parallel to the IO ports of the Jennic, there is a small driver for the Jennic which allows you to send/get bytes and read status info.
The board has space for one Jennic module, 3 status LEDs, on board 3.3v regulator and mini USB connector. The board is fully powered from the USB. There is a 4 pin header to access the rs232 for programming. There are two push buttons to allow the module to go into programming mode.
I have routed the board so no traces are required on the bottom layer – only a ground plane it needed. This makes it much easier to make the PCB at home as only one side needs to be developed (you still need double sidded copper though). There was one wire which I could not route, and did not have time to fix (the yellow wire in the pic!)