If its someone at college (say, at a party), I say that I play with my iPhone all day. If they ask for clarification, that I make iPhone applications. Usually at this point, whoever I'm talking to will say something like "So you write stuff on the computer for the iPhone?" and I nod.

The alternative is usually "Software Engineer" or "iPhone Developer", depending on where I am and how specific I need to be -- general description of what I do vs what my current job entails.

Or, if I really don't feel like talking, "student" shuts most people up pretty quickly.