One thing to consider is that a console will typically sell somewhere between 40% and 60% of its yearly total in November and December; this means that if you're averaging 250,000 units per month (Jan-October) you would expect sales to be somewhere between 4 and 6 Million units. It would not be unrealistic to say that if there was no shortage the Wii could easily average 400,000 units per month, this would mean that you would expect yearly sales to be somewhere between 6.5 and 10 Million units.
The Wii should be able to maintain this pace (at least) until 2009 simply because it will be the only one of the consoles which is selling at a truely mass market price; if Nintendo is successful at expanding the market then they could easily surpass their estimate.
