Developing iPhone applications is one of the hottest skills you can have these days. I can’t tell you how many regular programmers I have met who have either quit their jobs or on their way after their first or second iPhone applications. Now, you’ve got to have a viral application that sells, but if others can do it, why can’t you?
One thing that you shouldn’t do as an iPhone developer is copy a game that is already been created by others (especially large companies such as Nintendo). You may think that you can borrow ideas and nobody would care. But in reality you are going to open yourself up to nasty letters and possible legal action (though more likely you are going to have your game removed from the iTunes). You can always learn the hard way, but why spend hours and days copying others when you can come up with your idea and avoid trademark infringement.