Keynotopia is a pretty good mockup alternative for iPhone and iPad apps. It's not an application per se, but it's a set of templates for Keynote (or PowerPoint or Open Office). You export the result as a clickable PDF which works in a variety of viewers like GoodReader. http://keynotopia.com/ You can get pretty nice results that are somewhat interactive on the device itself.
Just to give you guys an idea of what I've been thinking of, here's a screencast of a little prototype I've been working on in my spare time:
http://screencast.com/t/tWUPNgGht2
The UI totally sucks and it isn't well designed at all. There's no sense of clickable screens, no sharing, no ipad, no retina, and no concept of a wire (only a mock). But the germ of the idea is there.