KenF:
Are you familiar with the Magic Lantern Project? I think you'll find that you can achieve everything you want with that, without any extra hardware (except an SD card).It was actually a spin off from the film industry and offers more features than you can shake a stick at. Even if you still opt to go with the arduino, I think you'll find that magic lantern will offer you more control than you'd have believed possible.
I was about to post the exact same suggestion. Magic Lantern is a lot better than the original Canon firmware, and the chance to risk damaging the hardware is nil.
If Magic Lantern wasn't a choice, then using an Arduino would be a great choice, but since there is Magic Lantern for the T3i (aka 600D), using an Arduino is overkill.