Eliot from Hackaday points to the iPhone-dev team's presentation at Chaos Communication Congress, in which the history of iPhone hacking–and its healthy state–is detailed. According to the gents behind it, more than a hundred users at Apple itself like to pwn.
It's an hour long and I've only listened to 10 minutes, but it's good stuff, full of interesting technical details. This of course assumes that you are interested in details of how USB connections are multiplexed by OSX Lobotomized Edition, etc.
25c3: hacking the iphone [Hackaday]