Very easy: Get a FTDI TTL-Serial to USB cable or breakout-board (eg: http://www.sparkfun.com/products/9716 or http://www.sparkfun.com/products/9718 ) and connect it to your M8RXD, M8TXD, DTR (Pin 4 on the DB9-connector) and ground. That's how the Pro Mini does it. For inspiration get the schematic of the Pro mini and look how JP1 (FTDI Basic) is wired up. You have all the necessary signals on your Arduino.
Korman