I'm really not sure if the PS3 will ever pass the 360 in worldwide sales. If you had asked me the question at the beginning of this year, I would have said yes for certain, but now I don't know. We're nine months into this year and the PS3 has made essentially zero inroads into the 360's lead; the two of them are pretty much right where they started, with the 360 having a lead of 6+ million units. That doesn't exactly inspire confidence in me as to the long-term chances of the PS3 surpassing its rival.
The PS3 will not pass the 360 in sales this year, that's absolutely clear. The 360 will probably increase its lead this holiday season, if anything. 2008 also looks highly unlikely. If the 360 stopped selling right now and never sold another unit, the PS3 still would not pass it at its current sales rate until late 2008. (PS3 is doing about 100k sales/week; it's behind by 6.32 million units, so that would take 63 weeks to surpass - November 2008!) Since that's obviously not going to happen, we can pretty safely assume that the crossover won't take place in 2008. Even if PS3 sales DOUBLE, in every market and every region, it still won't catch the 360 in 2008. Sorry, Sony fans, but it's not going to happen next year.
That leaves 2009 or later, which are so far in the future at this point that it's almost impossible to forsee. I really don't know what the market will look like 2 years from now, which is why I'm just going to make a random guess that there's a 50/50 shot of the PS3 eventually surpassing the 360. I really don't think that the PS3 will ever surpass the 360 in the American market, so if the crossover takes place, it will have to be on the strength of the Japanese and European markets.
Most of those arguing for a humongous PS3 sales surge are listing various games that will come out in the future. But, if you've been watching the charts here for a long period of time, you should be aware that software titles rarely have a significant impact on hardware sales. Usually sales surge for a week or two, then return to normal levels. No matter how great game ____ may be, it won't suddenly transform a struggling console into the market leader position. (Just ask the N64!) Crunching numbers may be a less interesting way of predicting future sales than listing exclusive games, but it's a lot more accurate.