PHILADELPHIA -- Like many teenagers, Ari Weinstein spends his summers riding his bike and swimming. This year, the 15-year-old had another item on his to-do list: Foil Apple Inc.'s brightest engineers and annoy chief executive Steve Jobs.

Ari is part of a loose-knit group of hackers that has made it a mission to "jailbreak" Apple's iPhone and iPod touch. The term refers to installing unapproved software that lets people download a range of programs, including those not sanctioned by Apple.

Since...