Realistically, you don't typically see a dramatic shift in popularity for a product; as a result the worst case for the Wii would be its sales would drop by 20% each year, and the best case for the PS3 would be its sales would increase 25% every year. If you assume that the Wii's sales for 2007 will be 17 Million, and the PS3's sales will be 7 Million, this would result in the Wii having sold 61 Million units at the end of 2011 and the PS3 having sold 59 Million units.
The best case for the PS3 and the worst case for the Wii still has the Wii leading by 2 Million units when this generation is (more or less) over ...
I don't think this is going to happen though being that Wii Fit after 2 weeks on the market has joined Mario Party 8, Super Paper Mario, Super Mario Galaxy, Dragon Quest Swords, and the Legend of Zelda Twilight Princess in outselling every PS3 game in Japan. The more I hear about this game, the more it reminds me of the begining of 2006 ... right before Nintendo released Nintendogs, Brain Training and Animal Crossing in Japan everyone was claiming that the DS' lead was soon to be over and people would realize how much more important graphics are
