CGI has the right idea, look at DS for example, usually systems peak in their third year but DS is on it's way to having it's best year this year which is also it's 5th year.
XBOX 360 looks like it's having it's best year too, in it's 4th year. This generation is like none other and you just can't go by past trends to make a case any more.. The PS3 has a mass market price point now and a very good back catalogue of exclusives along with many great exclusives coming in the next year. Blu-Ray, Wi-Fi and free online are all aspects that put it ahead of the 360 and will mean the added cost is negated if consumers see value in these things.
I think 2010 will be the PS3's peak year in terms of hardware sales but you just can't tell any more. It'll be an interesting few years that's for sure!