The iPhone APIs are written in Objective-C (some are just plain C) which is a nice language with nice and interesting features. If you have to deal with these APIs anyway, using another language wouldn't change much. You will have to create objects in the same way, send them messages in the same way, implement the same callbacks etc. Code is not much terser or easier to write with these bridges since you're not abstracting much, if anything.

Performance will be affected, heavily, since you're running everything on top of an interpreted language (with garbage collection) which then must call the equivalent Objective-C function, for every single line of code.

Moreover, I don't believe that any of these wrappers (for Lua or any other scripting language) will ever cover all the APIs available on the SDK. So, what happens when I need to call one of these? What if I need to use some other library written in c, or c++? And what if I've use this code to write some apps, the projects stops being maintained and the API changes with a new release of the OS?

What does make sense instead, is to use a language like Lua as it is intended, embedding it in your app and writing only the interfaces for the parts that should interact with it. For instance, if you're writing a game, it would make sense to embed Lua to code the game logic (which would be completely independent from the iPhone specific stuff).

Just my two cents.