Some people use a passcode on their iPhones simply to prevent their kids from mucking around with it or accidentally calling the police. Others use it for actual security reasons — because there's private information on your phone that you wouldn't want a stranger to have access to. If you're in the latter group, hopefully you use a passcode that isn't easy to guess.
Daniel Amitay, developer of the Big Brother Security Camera app (now removed from the App Store), added some code to the app to record user passcodes anonymously. Here are his findings.
Naturally, 1234 is the most common passcode: mimicking the most common internet passwords. To put this into perspective, these 10 codes represent 15% of all passcodes in use. Most of the top passcodes follow typical formulas, such as four identical digits, moving in a line up/down the pad, repetition. 5683 is the passcode with the least obvious pattern, but it turns out that it is the number representation of LOVE (5683), once again mimicking a very common internet password: “iloveyou.”
The passcodes are actually those entered on the Big Brother Security app, which shows a similar interface to the the iPhone passcode screen. Basically, the app replaces the iPhone passcode screen and takes a picture of hooligans who enter incorrect codes. So there's an assumption that the two correlate, but the assumption seems reasonable.