Of course it will slow down, the question is when.
The previous Nintendo consoles, N64 & GC saw peak sales in their first full year on the market. Wii is seeing improved sales in 2008 compared to 2007. which is the analogous year to 2002 for Gamecube and 1997 for N64, but sales are up in 2008 vs. 2007, despite not meeting demand in the west. Given that sales are up year over year, but still limited by supply the peak will probably come next year. I suspect PS3 will be peaking around 2009 as well when the biggest content is all out and the price is $250-$450. At the same time its not like price always boosts sales enormously, see Gamecube and even Xbox to look at that.
I have Wii & PS3 peaking in 2009, at ~30 million and ~15 million units, with the Wii decline from the peak softened by price advantages, and the PS3 decline from the peak softened by the advanced tech. Xbox 360 is sort of caught in the middle which is why the early lead gave way to Wii and probably will give way to PS3 in the second half of 2009 or early in 2010.
People are difficult to govern because they have too much knowledge.
When there are more laws, there are more criminals.
- Lao Tzu