It's pretty common practice in gaming for a company to, at the very least, have some kind of R & D going on for the next system the second the current console hits the market. So, basically, Nintendo was working on Project Cafe in some capacity the moment the Wii launched.
When they get serious on developing a new console, factor in some timeframes:
The system will have to be largely completed for a year to a year and a half (or so) in order for development teams to craft games for the system. Prior to that, maybe one or two years (or more?) of solid hardware engineering and firmware programming to get the system assembled.
This Sony announcement mirrors the ad that was discovered (by either IGN or GameInformer) that hinted Microsoft was hiring for positions related to "hardware development." If Microsoft and Sony are now beginning in earnest to develop their next systems--they probably won't be out until 2013 at the soonest. Which sounds just great to me. If the SNES and PS2 could continue surprising us 5 or 6 years into their cycles, then the Xbox360 and PS3 surely have a lot of surprises yet in store.
I don't think we've exhausted this generation yet. Well, maybe the Wii, but not the Xbox360 or PS3.