This happened to me last week. Mine was a corrupted picture file on the SD card after I installed an addon for LauncherPro.
Turn the phone off, remove the battery, remove the SD card, put the battery back in and turn the phone back on. Try opening Gallery, and it it opens then its something on the SD card. You can just slide the SD card back in without turning off the phone from here (there is a way to pull the SD card without turning off the phone, you need to unmount it from settings, and slide the card out carefully, but pulling the battery is safer).
If you don't have a card reader on your computer attach the USB (can't remember is Macs have USB or just firewire, if not find a PC), but make sure you move all apps off the SD card back onto the internal memory. Transfer the files over and reformat the SD card in the phone like mentioned earlier. That fixed my problem.
Selex