I put down 4th quarter 2008 (December) for the time PS3 to overtake the 360. Half way point of the year there is over a 5.5 million gap a further 5.6 million more PS3's than 360 sales would have to be sold in 6 months. The 360 would have to stop selling for that to happen.
Realistically I believe it will most likely occur in July 2009, when PS3 overtakes the 360 in total worldwide sales. Assuming the PS3 outsells the 360 in HW sales by over 100 000 weekly for the next 52 weeks. Taking into account the holiday periods and weeks where 360 manages to get close to the PS3 weekly sales.