Problem is you ignored a lot of factors in your analysis like holydays for example.
In terms of sales holydays are like adding a quarter or an extra 14 weeks to a year. Take those into account and already the date you mention has shifted.
Personally I believe the PS3 will overtake the 360 sometime in 2009, most likely Fall or Holydays 2009.
Besides you have to consider more than the current sales but the pattern for the generation.
So far for the 360 we have year 3>year 2>year 1.
However in year 3 there starts to be a few countries where 360 sales are bellow year 2 sales ( Japan, and France so far).
PS3 is doing year 2> year 1 and it's a fair assumption if it follows the 360 pattern that year 3 will be above year 2 ( market sure isn't saturated in any segment anywhere).
It's going to depend a lot on wether the 360 can sell more in 2009 than in 2008 or will in fact sell less...
PS3-Xbox360 gap : 1.5 millions and going up in PS3 favor !
PS3-Wii gap : 20 millions and going down !
