Statistically the PS3 can overtake the 360 by the end of this year. The PS3 sold 5.5 million more units by the end of 2009 than the 360 sold by the end of 2008, this even with a delay for PAL regions and other regions and higher entry pricing. That's about what the 360 sold its first year, the PS3 just needs to sell more than the 360 outsold the PS3 from release from now (minus this headstart, which is already been taken care of).
The PS3 slim at the moment is tracking to do about 15 million sales a year. The new control methods, the rumoured 360 slim, price drops and Gran Turismo 5 may have a profound impact of events though.