Memory management (ARC vs retain/release cycle)
Notification Center vs Delegates, what they are, when to use which
Some basic UI stuff
Obviously some basic objective-c syntax
How would I prepare? I would look at the type of apps they create, or I might be working on and start trying to duplicate at least one of them. For instance, if they have a popular barcode reading app, create one yourself, maybe even add an extra feature. Show it to them. They'll see it and know two things a) you're capable of creating it b) worst case scenario you can add features to one of their products. aka, you'll never be completely worthless.
How would I prepare? I would look at the type of apps they create, or I might be working on and start trying to duplicate at least one of them. For instance, if they have a popular barcode reading app, create one yourself, maybe even add an extra feature. Show it to them. They'll see it and know two things a) you're capable of creating it b) worst case scenario you can add features to one of their products. aka, you'll never be completely worthless.