Both Microsoft and Sony still believe that their respective consoles have plenty of life left in them, but that doesn't mean they aren't actively planning for the future.

Develop has a report from a source that says that work on "several" PS4 projects has begun at unspecified Sony first-party studios. These aren't games yet, per se, because their concepts haven't been green-lit  for full production. Particularly because the PS4's tech specs, etc. haven't been decided yet.

Develop has also heard from various sources that Microsoft plans to bring out the Xbox 360's successor, the so-called Xbox Next, in late 2013. The system would be fully debuted at the E3 convention that same year.

From everything we've heard on our end, these timetables seem very reasonable.