I believe they can support PS3 until very well 2015. They're still supporting PS2 aren't they? Both PS1 and PS2 were sweeping the floor with the competition when their successors were launched, roughly 5 years after release. It won't be surprising when the PS4 finds its way into stores 3-4 years from now. Sure the PS3 will not have probably recouped the loss on investment Sony has made with it by that time, and it will certainly not have half the marketshare they had during the PS1/PS2 era, but even that is little likely to mean they won't continue to fully support the PS3 (and third-party developers too) after that.
It's said and thought by many today that Next-generation "consoles" won't be as "hardcore" oriented or the tech powerhouses that would mean the continuation of the old "take big loss on hardware-even debuting with steep price-tag-model" So I'd say it's safe to believe "Next-Generation" hardware (which should specialize in things like community and interactivity (ie. waggle, touch screen) among other things (no clue yet), will coexist with current generation consoles. Who knows, in 4 years we could still be playing Xbox 720's games on X360, and PS4 games on PS3 (or VERY good ports of the next-gen ones), but the attractiveness and motivation to jump to the new platform would be all about convinience, commodity and added value, so much provided by the hardware, its design and its OS rather than what it can offer developers in terms of chips and bits.