Here’s a link to a little article I found about getting the Xerox Phaser 3123, 3124 and Xerox Docuprint 203/204a’s working on a Mac (including Lion).
Basically: the Xerox printers are just rebranded Brother/Samsung printers, so you can go to the Brother/Samsung website and download the drivers to make it work.
The Xerox Phaser 3123 and 3124 are the same as the Samsung ML 2510.
If you’re using the network on the 3124, then it just works as a generic postscript printer.
The Xerox DocuPrint 203a/204a use the Brother HL-2040 driver.
EDIT: If you’re looking for a qualified driver for the Xerox 204A, use the Brother HL-2040 driver linked above. The driver on the the Xerox site for the 204A is not a Windows Qualified driver and was having issues with iPrint for us.
Another Edit: If you’re having issues with the Xerox 204A printers, you can do a full factory reset on them by doing the following:
1. Turn the printer off
2. Hold down the Go button and turn the printer on. The lights will go solid after a few seconds with the Ready light off.
3. Let go of the go button, then press it 6 times. The printer will then start normally and be reset.
I got this info from the Brother HL-2040 tech manual here (this doubles as the Xerox 204A Tech Manual too): http://welcome.solutions.brother.com/BSC/Public/files/dlf/doc000360/HL2070N_NUG_ENG_1.pdf
If any of this has helped you, please leave a comment with how things worked out for you!