These instructions are better than the Adafruit tutorial. It uses the Mac OS terminal and the driver installation is different in Lion.
http://svbreakaway.info/tp-raspi.php
Combine with these instructions for using Minicom to test the port
http://www.hobbytronics.co.uk/raspberry-pi-serial-port
I was able to use cool-term and mini-com (with local echo set on each one) to have a 2 way terminal session at 9600 baud.
Next – we’ll try to use the serial port to send RTTY using the Arduino method.