A newly discovered iOS security flaw lets you easily bypass the iPhone 4's passcode to view the camera roll.
According to iPatch, simply popping out the back cover and disconnecting the battery for a moment will give you access to the camera roll upon reboot. You also need to make sure the SIM card isn't inserted so that phone doesn't set the correct time.
iPatch claims they haven't tried this with an iPhone 4S so its unclear if that device is affected as well.
Recently another bug was discovered by iPhoneIslam that lets you bypass the passcode for access to contacts and making calls.