I know there have been several people who have been baffled by the seemingly impossible task of removing large amounts of photographs from iDevices, myself included, so I've done some digging.
This was tested on an iPhone 4 (stock) and a PC running Vista SP2 64 bit.
Plug your iDevice into the PC using USB. In Windows Explorer, right click on the iDevice, and select "import pictures". A box will appear on the screen. Click on "options", then check "Always delete after importing". Click "Import".
The photographs and videos will be imported to your PC, and then Windows will delete the photographs from the iDevice. After the deletions, I needed to reboot my iPhone, but the pictures and videos had gone.
Hope this info is useful for other members.