Here is a view of the circuit tossed on a slab proto board. It is powered by a ~2m 4-wire phone cable from the wrt54g. The MAX3232 translates between RS232 and BCM4712 3.3v levels. I didn't bother drawing a schematic -- just follow the proto board connections to replicate. The datasheet corresponding to the Maxim part used will answer any connection questions.
http://www.seattlewireless.net/images/uploaded/97674.1105535857.jpg
The transceiver is a MAX3232 though similar Maxim 3.3v-able devices will work (pinout will likely be different). All charge pump caps are 0.1uF (yellow). The electrolytic cap (blue) is 4.7uF (non-critical) but observe polarity. The green led serves only as a 3.3v power indicator (optional) current limited via a 100-ohm resistor.
http://www.seattlewireless.net/images/uploaded/97768.1105535900.jpg
The wrt54g side of the phone cable. Wires are soldered to a single 0.1" socket header. Note the PCB pin header has the key pins snipped off as should the socket header have the corresponding locations filled with something solid (broken off toothpick will do) to prevent a reverse connection. This is a good idea as 3.3v and ground pins are at opposite locations on the connector.
http://www.seattlewireless.net/images/uploaded/97848.1105535934.jpg
