We've tested it out and it's easy to set up. First, head to Settings on the PS3 media bar, then Accessory Settings and Manage Bluetooth Devices and finally Register New Device. After that the PS3 will scan for Bluetooth controllers, so to make your DualShock 4 show up hold the PS and Share button at the same time. After a few seconds the controller's light bar will start blinking, and then it'll show up on the PS3 screen ready to be connected.
A few words of warning: This will de-sync your DualShock 4 with your PS4, but re-syncing it is simply a matter of connecting the gamepad to the PS4, turning the system on and then pressing the PS button. Also, our pal Engadget reports compatibility with various games is "somewhat spotty," so don't be pushing your DualShock 3 off a cliff just yet.