Osman Eralp of Ozbots writes:
When I was building my latest Chumby-based robot, I didn’t need the serial port, but I did want PWM signals to control the robot’s wheel motors. Fortunately, the processor pins that are used for the serial (UART) TX and RX pins are multiplexed with PWM outputs. With the proper commands, you can tell the processor to output PWM signals instead of serial TX and RX.
His process involves four steps which may be accomplished through one of two methods: either running regutil (a command line utility), or adding C code to your robot program. When completed you will have changed the function of the serial TX and RX signals to PWM outputs.