Granted, a leaked case (and clearly not an Apple-made one) isn't the best evidence, but the source has a reasonable track record when it comes to Apple's doings - Sonny Dickson.
It joins the left-hand-side-mounted Action Button as a new control for the iPhone and is a very un-Apple move. It's expected to mimic the soft shutter button on a typical camera.
[
add
]
[
|
|
...
]