According to an article on Cnet, there seems to be a problem with some iPhone OS 3.0 Camera Roll’s on some iPhones as a few photographers have reported some picture taken on the iPhone along with screen captures vanish from the iPhone photo library.
Apparently the bug happens when the iPhone generates photo names IMG_10000.xxx and above, where the xxx means a jpg, png, or mov.
Once the iPhoner hits the 10,000 point the Mobile Photos application will no longer show additional media files, they haven’t been lost, it just doesn’t display them.
