After I updated my iPhone’s iOS operating system from version 5.1 to 5.1.1 I soon discovered that all my photos and videos were gone (no longer visible in the Photos app). After a bit of troubleshooting I figured out that all my photos & videos were in fact still on the phone but the media library index had become corrupt or something alike. By deleting two files on the phone (by using iExplorer), rebooting, opening the Photos app, and waiting a few minutes the media library reindexed itself and everything was back to normal.
I followed these instructions but the files .MISC/Info.plist and PhotosAux.sqlite weren’t on my phone so I just deleted DCIM_APPLE.plist and Photos.sqlite which turned out to be enough. Thanx mbentley!