I did see a really nice project where a guy had built a single box that could be switched to trigger a camera based on sound or light, it was in a tough looking box so great for using 'in the field'
There are at least two commercial projects that use Arduinos to trigger a camera based on sound, light, or other triggers (triggertrap.com, cameraaxe.com). Both have fully build versions, as well as Arduino shields (note, the Triggertrap shield is not yet shipping).
There are also some companies that use cell phones instead of Arduinos, using the sensors in the cell phone. And another company that produced its own unit before Arduinos and Cell phone triggers (Mumford Time Machine).
There are several other projects that I've bookmarked that were not commercial (at least initially, you can find the original post from the guy who founded cameraaxe).
However, most of these devices only work with cameras that have electronic or infrared shutter releases. For the electronic shutter release, there are 3 wires (ground, focus, shoot) and you need to connect ground+focus to get the camera to focus, and either ground+shoot or ground+focus+shoot to fire. In these, you plug the Arduino's signal into an optocoupler, and connect the ground and focus/shoot wires to the output of the optocoupler. For infrared, you use an IR led, and send out the appropriate sequence.
If your camera doesn't support either a wired or infrared shutter release, your options are to hack open the camera and add an optocoupler connection to where the shutter is to take the picture, or use a servo to press down a button. I would imagine the USB camera doesn't have a button like a normal camera does however.