So far this year, PS3 outselling 360 by about 60k/week. Now that Blu-Ray has officially won, that should go up a bit. If it averages 70k/week, then PS3 is on track to pass 360 around the end of 2009. Right now, that is looking very doable.
My prediction (Jan 15th, 2008), total sales by end of 2008:
Wii - 35 million
X360 - 25 million
PS3 - 21 million