Technically, they stole the idea, but on a lesser scale since the PS3's motion sensor is a lesser technology compared to the Wiimote's motion sensing capabilities.
To deny any such influence is real BS. Yeah, they had the idea and the patent for motion sensors before Nintendo unveiled the Wiimote, but I seriously doubt they had any thought of using it in the PS3 controller. We all know Sony is the type of company that, if they have technology planned, they will instantly show it off. Look at E3 2005, remember how fast they were to relinquish all the specs and information they had on the PS3? No secrets left after that presentation. If they had such an idea for the Motion Sensor for the PS3 controller, they would have said it was in the Boomerang.
Speaking of the Boomerang, what happen to it? They seemed pretty intent in convincing gamers it was a good controller, addressing the proposed comfort issues. You can't say after sugarcoating the Boomerang controller that they all of a sudden want to revert back to the old design out of nowhere and with no intention of upstaging the Wii controller.
Note they also delayed the console from the proposed Spring 2006 release date shortly after TGS 2005, excusing it as Blu-ray problems. They also told the Warhawk developers about the motion sensor just 3 weeks from E3, which makes it increasingly obvious it's a means to 1 up Nintendo; if they had any intentions for it all along in the controller, they would have hinted the Warhawk developers earlier, like normal. Plus, they can't use Dualshock anymore due to a loss in court against Immersion Corporation. They lost their appeal in March, so it's really possible that they spent less than 2 months on the motion sensor idea.
So yeah, along the lines they copied Nintendo as a means of competition, but no so much to complete detail due to their snap desicion. Due to their release date for the PS3, they have time to fully duplicate the Wiimote though.