Is there an app for iPad to assist in wireframe design for iPad and iPhone apps?
I don't mean actual development, just placing standard widgets and the things you usually do with photoshop and associated kits.
Blueprint is, in my opinion, the most powerful and most useful, and comes the closest to the promise of Briefs. You get customizable standard controls, it's easy to work with, and most importantly it allows you to set up actions associated with taps, swipes, and rotation. The key is that not only can you work out a complex app interface, but you can then distribute the completed mockup so that other people can play with it using the free Blueprint Viewer app. Super, super handy.
The only downside of Blueprint, imo, is that you can't easily create mockups that don't look like a finished app. When working with outside clients or with semi-clueless management, if you present something that looks finished they'll sometimes confuse it with the idea that the app actually is close to being finished. Rougher wireframes are also useful for getting people to focus on the functionality and not how it looks. Still, a great app, the best imo.
Now to the also-rans:
Interface HD holds a lot of promise, especially the idea that it can generate some basic Xcode template stuff, but it's very unfinished and doesn't feel very polished. Definitely more promise than followthrough at this point.
UI Sketcher is designed for rough sketches and can be used for a variety of projects, not just iOS design. Unfortunately that lack of specialization means that it's not actually very good at iOS app mockups. It's so crude, really, that I've never been even slightly satisfied with a result. Maybe if I used a stylus.
iMockups and SketchyPad do have more iOS-focused controls and options, but both fall prey to the UI Sketcher problem of being designed for other things, too. Additionally neither feels sufficiently complete or polished, and everything is harder to do than I'd like. I've used iMockups to create something I actually presented to clients, but felt the need to constantly apologize for the ways the wireframes weren't quite like I intended the app to be. SketchyPad exports to Balsamiq Mockups format, so if you're already a big Balsamiq fan then it might be a good match for you. Both are definitely a step up from the looseness of UI Sketcher (or any other sketching app), but just aren't good enough to actually use for real work.
I've not used Mocking Pad, but it appears to be nearly identical to iMockups.
I've adored Briefs app since running across it a few years back. Nothing comes close to the power and design of this app. It for a long time was probably too powerful (sadly) to ever ship on the iTunes store - but the app has finally launched on the Mac and iOS App stores.
WireframeSketcher comes with dedicated stencils for iPad applications. You can use links for simple interactivity. You can even test your prototype on iPad by exporting it to HTML that you can put online via Dropbox.
I have found AppCooker to be robust enough to create a mockup with 40 > screens. You can specify your own tappable areas and links between the screens so that those mockup screens come to life.
However I'm really interested in finding alternatives too: AppCooker is great. Don't get me wrong. However I really wish that it had the ability to handle different layers. It requires a lot of cloning and violates the DRY principle: if you want to change an element that's already been copied into 5 different screens, well, you gotta make the change 5 times! Not fun.