I was going over Brian's PWM article, when it hit me, about why a driver is reffered to as being high-side or low-side.
So I found this interesting chip:
Maxim's 1614 (PDF, 8 pages). I'll review it later, to see if it could drive the MOSFET of the 12v line.
edit:
High side driver is for when a MOSFET is hooked up between the high voltage (i.e. +12) and the load.
Low side driver is for when the MOSFET is hooked up between the load and ground.
Since the ground is common in a PC for all voltages, it's not possible to setup a low-side MOSFET.