I remember when Sega released the Saturn. It was without any hype at all and it just kinda "came out" in the middle of summer. no one even fucking new it was out and then all of a sudden Circuit City had one for like 500 bucks.
Wii will look like the early winner until maybe early 2008.
360 will coast and gain share this gen but will not overtake the other two in the end of this gen
PS3 will have a slower start but will likely be the leader by the end of this gen.
Neither will die and go the way of the Dreamcast or Saturn and if any of them do, they better at least have libraries equal to those two(2 of my all time face systems) when they go.
Sony isnt going away but their video game development most likely will. Development of the PS3 crippled that leg badly, the stock took huge hits for it too. I doubt they get involved in the next generation development. It's going to just take too much money to develop for a company like Sony to absorb.