I've got a cheap keychain digital camera ($5), that I bought a while back for a similar purpose, but haven't gotten around to buying an SMD AVR chip to stuff in there. Arduino is a huge over-kill for this, would stick to a single chip. My camera has two buttons, but pretty sure it can be switch directly from the I/O pins. I'm looking at a motion detector, but the timing is similar to snap the picture. I keep putting this off, mostly because the camera's memory is lost, when the battery is drained, and haven't figured out how to prevent that just yet.