I'm making a fruit salad with kiwi, orange, grapefruit and banana. Will the acid in the citrus stop the banana from turning brown? Ideas?

an extra sqeeze of lemon never hurts, and tastes good too!

I would also suggest cutting in the bananas close to the time you plan on serving. But it might be nice to give them a bit of time so that all the flavors meld. What I would do is coat them in some orange juice when you are ready to toss them in. Doing so will provide extra" brownness protection," and since you;ve got oranges in the mix, the OJ is a natural match.

I agree about bananas getting mushy. How about switching in a pineapple instead?

I agree with latoscana. They'll be protected from browning, but I suspect they'll also go mushy on you, which is probably not the effect you're going for.

It should but best if you add bananas just before serving.