I design user interfaces for Andoid and iOS apps and use a prototyping (can do simple wireframes and full mockups) tool called Axure
I've tried quite a few tools in the past and I have found Axure to be the best so far, it's well supported with lots of tutorials and is quite versatile in what it can produce.
It allows you to design your layout/wireframe screen by screen and then 'publish' it as a mobile website optimised for iPhone screen sizes. You can open that link in your mobile browser on your device and navigate through your wireframe screen by screen. You can also save the web link to your home screen and set the app icon, so it does a very good impression of being an app.
What it doesn't do however is let you design actually on your iPhone (If that's partly what your asking?). You design on the computer and then send the designs over to your phone/tablet. I don't know of any tools to actually create mockups on the phone directly. Personally I would find it far to tricky to design anything in any detail on such a small screen, it would be very limiting. It's quick to get designs from Axure on your desktop to phone though. Once you've created a project you just hit 'publish' and then refresh the webpage on your phone.
For a better explanation of Axure's mobile capabilities than I can provide in a short post take a look at their iPhone app tutorials; http://www.axure.com/learn/iphone-app
Hope that's helpful!