Besides, Brawn will make Nico an offer he can't refuse...Brawn know if they take Nico and leave Fernando and Kimi at Ferrari and McLaren respectively, they get something of an advantage. There's little doubt McLaren and Ferrari will have some of the better chassis in 2010, and Brawn will have a tough time beating them. However, if Brawn have a clear 1-2 pairing in Jenson and Nico, they'll have no distractions to go forward in another title run, while McLaren will have Kimi stealing points from Lewis and Lewis stealing back from Kimi (as well as Kimi bringing team morale down), just as Ferrari will with Massa and Alonso. Brawn can't let Nico go to McLaren or they're going to have a tougher battle.
Rosberg will be excellent at Brawn, and I think there is a good chance him and Button would take points off each other as well (I don't see who would be the better driver there). Button isn't the number one either, the team may prefer him to Barrichello because he's less of a whiney bitch (and of course, a better driver), but they have been allowed to race each other all year long.
The Mclaren team like Kimi, he fitted in well there and now that Ron Dennis has gone it could work even better. Kimi is not the kind of guy to cause friction in a team even if his team mate beats him (see Massa). So, all in all I think the line ups mentioned (Alonso, Massa + Nico, Button + Kimi, Lewis) would be pretty good for the future of F1.