YouTube user Udetto tried out the PS4 controller with his PS3 and saw that it worked -- somewhat.
Testing out a couple games with the PS4 controller plugged in to the PS3, YouTube user Udetto found that the PS4 controller is compatible with some PS3 games. The catch: the controller has to be wired. The moment the controller is unplugged and wireless activates, the PS3 will not recognize it.
Udetto played Urban Trial Freestyle, a dirt-bike videogame, with the DualShock 4 plugged in via USB, and he reports having no problems with the controls. "Works a little bit too good," he joked as he went too front-heavy in a jump. He said the controller worked perfectly.
Udetto also tried playing FIFA 14; however, because the game requires the controller to be wireless, the PS4 controller could not be used. "The menus were fine," he said. "As soon as it came to 'wireless connection required,' it didn't work." Navigating the menus of the game with the controller plugged in still worked.
After receiving many questions in the comments, Udetto appears to be looking more into what the controller can and can't do on the PS3. One commenter suggested paring the PS4 controller in the menu for wireless. Another asked how the Share and Options buttons work for the PS3 (rather than Start and Select).
Controllers are expensive. The DualShock 4 sells individually for $59.99. It would be nice if the PS3 controller would work in some capacity on the PS4.