As of iOS 5.1, there is no easy way to delete multiple photos from the Camera Roll on the iPhone, iPod Touch, or iPad. However, if you have a Mac nearby, you can simply use Image Capture to select multiple photos on your device and delete them instantly! Using your Apple USB sync cable, connect your device through the USB port on your Mac. Then open Image Capture and your device should show up. Then, use Image Capture with your iDevice in the same way that you would use it with your digital camera or other media device!
Hope this helps, if you have any questions, please leave a comment!