Sony Ericsson mobile phone, as F500i/k500i, T610, etc. may be connected to a computer with a serial (phone side) to USB (computer side) converter, like DCU-11 cable. The purpose is to transfer files (mp3, pictures, etc.) between phone or computer, or use the phone as a GSM ou GPRS modem. The only "little" problem is that Sony Ericsson only propose Windows tools and drivers.
The main thing to know is the converter uses a Prolific PL2303 chipset. Here comes the how-to about using DCU-11 cable with Mac OS X 10.4.x (it should work with Mac OS X 10.3.x).