Never.
PS3 is retreating upmarket, focusing on short-term profitability. It will always have the price disadvantage.
360 is focusing on the mainstream, looking for the next "Twister", rumors of a new controller, and learning a lesson from Wii Fit.
Just focusing on current numbers, PS3 would take over in 2011. But I believe business strategy is a better predictor of success, so the company who is best at copying Nintendo will secure 2nd place. That company is Microsoft.