I'm sure there will be an app...either Apple will come out with an iChat app or instead perhaps they'll work with things like AIM to make sure they support it by launch day. It will primarily be the same as the camera on the Macbooks...launch a chat service and connect.
(In fact, I think iPhone-to-Macbook will be far more common a connection than iPhone-to-iPhone.)
But you raise an interesting point...when on the speaker phone on a connection between 2 iPhones, will a "show video" button appear on each phone?
I hadn't thought about that at all, but it's certainly possible.