In his last update, Daniel Amitay, the developer of Big Brother Camera Security (Free), added some code to record common user passcodes (he stresses that this is completely anonymous) for the app. Since the app's passcode setup screen is near identical to the phone's passcode screen, he opines that the collected information would reflect the actual iPhone passcodes users would use. He has published his findings on his blog.
Of the 204,508 recorded passcodes, 1234 is the most common passcode used. He also found that 1990-2000 are all in the top 50, and 1980-1989 are all in the top 100; which he interpreted as a 'subset of users that set their passcodes to the year of their birth or graduation'.
Do check out his blog for more details.