This is a cool app but why not just muck around with the iPhone App interface builder that comes with the SDK. It is basically a much better version of this web app.
First, those mockups are not supposed to look "real". The idea is to give somebody something that very obviously is not anywhere near a finished product so as not to give the wrong impression of how "finished" the product is.
Second, it's easy to share my mockups since you can simply share the URL, and all changes will be synchronized live. So you can collaborate over the phone and see each other's changes (which is why I've made the thing).
