For me, the question is really how much will sales increase for PS3 when the price drops? It is selling between DC and GC levels right now at $600. Last generation sales (PS2) will end up at 125 million. Say the average price for PS3 from 2006 to 2012 is $350. Is that expensive enough to lose 85 million customers even as the market grows at least another 20 million users bigger? Remember, when sales do get under $400 or so, sales accelerate much more rapidly. A $50 price drop might be a 2% boost in sales for PS3, $100 off would be maybe a 5% boost, $150 drop would be a 17.5% boost, a $200 drop would be like a 70% boost.
Basically, if PS3 on average, cost $350 from 2006-2012, I would expect sales to be something like 2.5x the absolute lows of 2007 in the long run (from the lows of 40k/week right now to 100k/week).
I would say losing 85 million customers qualifies as a pretty solid worst case scenario. I mean that is essentially losing the combined userbase of the SNES & Genesis (as an eccentric comparison).
My main point is, I think after 2 years or so, when PS3 is cheaper, it will be selling faster than either GC or Xbox did because of a stronger presence in Japan & Europe.