Massa's contract at Ferrari expires end of next year. I doubt he'll leave voluntarily before that, because he has nowhere better to go. Ferrari's unlikely to pay Massa out unless they have a far better replacement, and it would be a brave/foolhardy driver who agreed to race in the same team as Alonso. Odds are then the Ferrari team won't change next year, but Massa's almost certainly a dead duck come 2013.
Alonso is toxic to his team mates, and he's locked into Ferrari for the next five years. That leaves Ferrari with an interesting problem in 2013. It's hard to predict which top driver will be desperate enough to take the second seat come 2013.
RBR is unlikely to change. Webber and Horner are both making the right noises for a contract renewal. When Webber does retire, I guess in a year or two, there are so many good options for a replacement that it's an embarrassment of riches.
Hamilton can do no wrong in the eyes of McLaren. He might be eyeing RBR but he won't get a seat next year, and he's crazy if he thinks Vettel won't be their #1 driver when Webber retires. He's also crazy if he thinks the McLaren is not at least the equal of RBR now. Nor is he likely to rematch with Alonso at Ferrari; not after the disaster they had as a team last time. Therefore Hamilton's not moving.
Technically, Button is the better driver, and even McLaren might be forced to admit that soon; at least, they'll have to if they want more wins. Button would be an absolutely brilliant replacement for Webber, but he's probably one of the few drivers who wouldn't necessarily jump at the chance. I think he's found his natural home at McLaren.
The difficulty is to find good seats for Kobayashi and Rosberg. They've both more than earned winning cars. Rosberg I guess will stay with Mercedes, with no better option and enticed by piles of cash and his own team. Kobayahi is another who'd make a superb Webber replacement.
Further down the order it's pretty much randomness as far as I can see. I wouldn't be surprised if Virgin got the wooden spoon this year. Which would trigger a big reshuffle.
Heidfeld is a much better driver than his results have ever shown. He seems to have the mark of doom upon him. If he could produce some good results with Renault this year he might be picking up another seat, but if so it won't be with Renault. Sadly, I doubt Kubica will make it back, or if he does, it might be brief. There are *lots* of drivers to fill that seat.
The problem is there are too many excellent drivers, and not enough winning cars.