I was thinking it would be cool to have an app that can take a picture the instant a sound threshold is crossed. For example, snap a picture the moment a balloon is popped, etc. If the iPhone's camera isn't fast enough to do this, another idea would be to display a completely black display and to flash it white (or some other color) briefly on a sound trigger. Then, I could use the iPhone as a flash for my DSLR to capture images based on a sound trigger. I'm tempted to dust off my programming books and make such a thing myself, but I'm hoping someone else beats me to it.