32- and 64-bit PL2303 drivers for OS X 10.6 are available here.
You'll need to modify, as root, the '/System/Library/Extensions/ProlificUsbSerial.kext/Contents/Info.plist ' file after installing the driver to suit the USB manufacturer and device ID. For the ATEN UC232A, examples below appear to work for it (for me in 10.7).
To obtain the IDs, Wayne Roberts (who informed my via the SAGE-AU mailing lists), used 'USB Prober.app' which comes with the developer tools/XCode.
Modify the current lines of the XML:
# <key> can be the Hex values as 'Vendor'_'Product', Wayne thinks this is more cosmetic however.
# <idProduct> and <idVendor> should be the decimal of the respective values, as per USB Prober.app
Once you've done this, either restart the machine or run 'kextunload' and 'kextload' on ProlificUsbSerial.kext and it should show up as /dev/tty.usbserial.
** If you have installed the UC232A Drivers, either rm or mv the 'UC-232AC.kext' before the reboot to avoid conflicts **