I don't play very many games, but the ones that I do play I absolutely love. So for the next Batman, Uncharted, Assassin's Creed...if they're only available on the PS4 and NOT the PS3, I'll probably find myself upgrading.
I just wish there were more reasons to upgrade. My PS3 is one of the best purchases I've ever made, not because of the games, but because of the Blu-Ray player and WiFi capabilities.
That will happen pretty definitely.. why should a developer create games for an outdated console (even if it has millions of users)?
Given the totally redesigned internal architecture porting games wouldn't also be easy or cheap so once thr PS4 hits and companies develop games for it they won't for the PS3.
I guess the PS4 will also double as a home media center so playing Blu Rays, surfing the web etc should all be possible. Given recent reports though is that the new Xbox will focus very heavily on that so it remains to be seen which console will do this better.
I'm definitely buying one of them come Christmas.. it'll be down to available and interesting games and price.