PS3 will have the longest lifespan ever of any console before it successor will be released. I think it will have a 7 year life before PS4 comes out. The graphics revolution is over, and with add ons the PS3 will be able to tap into the interface revolution started by the Wii and DS.
End of 2009 Predictions (Set, January 1st 2009)
Wii- 72 million 3rd Year Peak, better slate of releases
360- 37 million Should trend down slightly after 3rd year peak
PS3- 29 million Sales should pick up next year, 3rd year peak and price cut