rant: I've noticed this since my ICS upgrade. My Gallery won't load new pictures copied or moved to folders until either the SD card is unmounted and remounted or the phone is rebooted. I tried clearing the app's data and reformatting the SD card and nothing has worked.
My final conclusion was to install QuickPic, which seems like a worthy substitute. It's just a shame to have to rely on a third party app for something that should be a basic smartphone function.