I've just implemented interactive transitions and am seeing an issue that's also present in your example code Pete. Just wanted to see if you had any thoughts before I open a radar...
When the second view is pushed, there's a nice animation of the navigation bar title into the back button (easy to see in the simulator in slow motion). However, when popping back to the first view controller there's a weird blurring instead. Regular navigation controller pops do the reverse animation and I'd have expected custom transitions to do the same.
Maybe I've missed something, but I've not seen anything in Apple's documentation. I don't think it's actually an issue in your code... but maybe I'm wrong!