This chart shows the 10 most common iPhone numeric lock-codes, as anonymously gathered by the Big Brother Security Camera app (now removed from the App Store). They represent 15% of all 204,508 passcodes sampled. I wonder how strongly they correlate with other numeric PINs, such as ATM/Chip-and-PIN codes.
Interestingly, 1990-2000 are all in the top 50, and 1980-1989 are all in the top 100. I would interpret this occurrence as a subset of users that set their passcodes to the year of their birth or graduation.