If you argue that the Wii cannot double its sales in Q108 versus last year, you are essentially saying that Nintendo cannot keep selling those 1.8 million consoles per month after Christmas at all.
I don't think that the Wii will catch up anymore in the US this year. I would expect that the gap between Wii and 360 will be around 2.5 million by the end of the year. In the first quarter it's very certain that the Wii will catch up by more than 1 million units (given how long the lines still are now), though exactly how much is really up in the air.
The releases of GTAIV or SSMB probably won't move a ton of extra hardware. A 100k spike is the best you can expect from a AAA gamer's game, and if this game improves the library of a console, in the long run, the average weekly sales of that console may go up permanently by a few k. Wii Fit on the other hand is a new genre (or new market, whatever), so if it's successful it could really influence sales.
Hardcore gaming is a bubble economy blown up by Microsoft's $7 $6 billion losses.
