Use ATX power source to power Raspberry Pi
I see no reason why this shouldn't work. ATX power supplies generate a relatively well regulated +5V rail with lots of power available. I am of course assuming your plan is to connect to the micro-USB power port (as opposed to the USB host ports).

The only thing to remember is to add a minimum load on the +5V rail to ensure proper regulation. A 22Ω 2W rated resistor to ground should be sufficient to ensure proper regulation.