App Layout is a tool to get you started.
App Layout comes with a variety of controls you can drop into an iPad or iPhone app design. And if you can't find what you're looking for in the built-in control set, you can always import an image of your own (so you can quickly "borrow" user interface elements from other apps). It's easy to select, drag and drop all your controls into place. And when you're finished, you can save or email your work of art.
OK, now for the warts... Saving/emailing is great, but how about reloading a project and letting me continue editing it? Any prototyping you do with App Layout has to be done in one session. There's no way to reload a previously-saved file and continue tweaking it. That's a huge problem. Also, there's no way to use typical iOS gestures (e.g., pinching) to resize controls. You're forced to press zoom/shrink or contract/stretch buttons till your fingers hurt. Why in the world didn't these guys implement a feature as standard as spread/pinch?!
All that said, I only paid $2.99 for the app, so I can't complain too much. It now costs $4.99, so I must have benefited from an introductory price (the app was just released on 8/3). For $2.99 it's not bad, but I think they could get $9.99 if they'd address these shortcomings.