After my gf upgraded to iOS4, her camera pictures disappeared. Seems like the discussion forums are swamped with people with the same problem. Here is the solution: (oh, she's running linux, so she are forced to run iTunes in vmware, and these instructions are thus for windows)
Fix iPhone iOS4 "empty camera roll"
iphone explorer (http://www.macroplant.com/iphoneexplorer/)
1) Connect your iPhone
2) Launch iPhone explorer and select the "/var/mobile/Media" as the path
3) Backup and then delete the following files:
4) Unplug your iPhone, launch Camera Roll and wait as it rebuild the database.
Credits: discussion thread at Apple discussion forum