Spycraft 2.0 has a bunch of subsystems for social combat based on the chase mechanic they developed in Spycraft 1.0. I haven't tried them, but they seem a little too mini-game-y to me.
My impression is that a lot of indie games blur the distinction between physical combat and other kinds of challenges, including social encounters. Risus is a pretty simple implementation of this idea.
Speaking of Burning Wheel, you can check out the Duel of Wits rules for free
here, though they only make a little sense to me, since I don't know the basic rules.
In general defense of social encounter rules: I GM'd a game with a great social roleplayer. The final encounter of the campaign involved a trial wherein the PCs accused an important noble of treason, and the back and forth between us was excellent. I realized, though, that I needed to wrap the encounter up or it would go on forever, so I called for a Diplomacy check. All that, leading to a single die roll, was very anticlimactic (and reminded me of
this). Maybe I could have handled it better, but I thought that some sort of mechanical support that would divide the encounter into discreet, intuitive, testable checks would have made the encounter better, without losing the great roleplaying that happened.