Facebook built a prototyping tool called Origami, and released it for free. It's how they mocked up interactions for Paper, their most recent app.
I'm pretty keen to install it and try it out, but I think you need an Apple Developer license to get it. I think it's about $100/year. I had one of these once upon a time, when I was experimenting with building iPhone apps, but it has since expired.
There's a good write-up of Origami here.
Has anyone experimented with this? I'd love to hear people's thoughts.