Well, currently Nintendo is forecasting 52m by end of March 09 , and they are also producing units at a rate of 2.4m a month (in financial statements)
So they are producing Wiis at 28.8m/year. Since we haven't hit peak demand yet (still sold out in the states, and it seems like it sells more each week in Europe, and Japan could take off again) I'm going to assume they sell out completely for the next 2 years.
So 52m + 28.8*2 = 109.6m units end of March 2011. So the system would have been on the market for 4.5 years, which it took the PS2 ~7 years to hit that same mark (granted, they launched in Japan before the rest of the world, but still, 4.5 years is extremely fast)
Of course, you then have to add on the typical fading of sales over the years, and if we assume it lives for 5 years before a successor, it should easily break the PS2's sales record.
Also, I personally believe they are going to up hardware production one more time so as to meet Christmas demand next year.