This sort of all makes sense if you think about it, and if the various rumours are to be believed:
McLaren's Vodafone sponsorship expires at the end of the 2013 season. Sergio brings Telmex to the table, meaning they don't have to renew Vodafone.
Sauber need money, so Gutiérrez may replace Sergio, also bringing Telmex backing. Gutiérrez isn't great, but it's all about the money isn't it. Alguersuari has apparently been quite confidently vocal about having a drive next year, so maybe it's at Sauber.
Lots of fairly reliable sources claim that Vettel at Ferrari in 2014 is a done deal. If that's the case, whoever's alongside Alonso will only be there for a year. That suggests that Massa will probably stay!
Lewis could be really good at Mercedes, who knows? It'll certainly be interesting to see him drive the car anyway. As much as I dislike him, he is very good!