Yup thats right, ThinkGeek.com have developed a joystick attachment for the iPhone.
Made from milled Aluminum simply stick the joystick to your iPhone screen and its ready to use. Once you are done simply take it off, simple. They can be purchased from ThinkGeek for a price of $17.99 each, which is expensive if you are planning to buy 2. So unless you are a keen iPhone gamer i doubt they are worth the money.