Interestingly my thoughts have been along exactly this line since the weekend. Until this weekend i thought Lewis would see his time out at McLaren but he clearly isnt happy and isnt supported by Whitmarsh. So......
Not Ferrari becuse Alonso wouldnt cope
Probably not red Bull-they have Vettel and they like a slightly slower second driver
Mercedes!! I think this could be a clever move. They look serious about joining the top three teams. Brawn knows how to win and they've invested in top staff. Lewis has a good relationship with Haug. And I think they'd manage him better.
He likes Rosberg, they're friends, so when Schumacher retires, that could work. It has a lot going for it.
A less likely scenario is lewis with Schumacher. Which I actually think could be a great combination. (my feelings towards Schumacher have mellowed since his return). They're both racers and Michael could help Lewis to turn the other cheek when the crap starts falling. Help him grow a tougher skin and not care. Because lewis does care and thats taken advantage of, even by his team.
He's not happy, he's not appreciated, he's not racing well. Whats to stay for?
Other than Mercedes I can see him doing a Montoya and leaving F1 altogether. Another great driver lost because of poor management and political nonsense.