Not throwing handhelds in there.. but i think it will go
PS2 -> Wii -> X360 -> PS3
PS2 8 years old.. still going fairly strong.. 2 years left at best.
Wii Dated internals, no HDTV, no hard drive
X360 Xbox3 Will come out, Following on from Wii2
PS3 Sony needs to make their money back, and it'll move into more of a media unit than a games console.