I was actually going to point that out myself ceres. Neither the PS3 or the Wii have seen a real holiday season yet. And the Wii may still be extremely supply constrained this holiday even with stockpiling.
With that said, the current "rate" is an incomplete analysis and if you evaluate it in that context it is still a valid Ω(n) analysis. But it is important to remember that a very large portion of sales come from these holiday seasons and in the case of the PS2 its best holidays were after price cuts in years 2, 3, and 4 iirc.
So with that in mind this analysis, while interesting, is lacking some vital data to reach its full potential. But it is something worth watching as it develops. The two key points for this analysis are going to be the next two holidays ('07 and '08) as they will give that incite into the Wii holidays sales that is lacking right now.
Edit: PS - For those who don't know: Ω(n)= Big-Oh notation for asymptotic lower bounds. In other words it means its a good analysis for the lower boundry of performance. Too much time at school perhaps >< Need to spend more time gaming..yeah thats it!
Edit2: If you're interested the upper bound is notated by "O", hence Big-Oh notation. To signify a tight match you use the character "Θ".