Users are reporting a bug in iPhone firmware 3.0 that prevents photos from appearing on the camera roll. Although image files are present on the device and can be copied with iPhoto, thumbnails are hidden on the camera roll.
The camera roll even counts the correct number of photos as it hides a portion of the files. Users have determined the problem arises when numbers in photo filenames exceed 10,000. For some reason the iPhone OS 3.0 camera roll does not display these files.