Perhaps this has been covered already, but I can't recall any mention of it in the tons of iPhone material I've read since the device was released a little over a week ago. Turns out that if you are using the iPhone as a phone (i.e., holding it up to your ear), it keeps the screen off to avoid draining the battery, and when you suddenly move it away from your head the screen automagically returns. Very nice.
The device's I-know-how-you're-holding-me, inner-ear magic, makes this an obvious and logical feature, but, as is so often the case, and in this space in particular, obviousness and logic don't always find themselves on the right side of fruition; when they do, as here, it's almost shocking. Hopefully Apple will soon have us expecting these sorts of things from our mobile devices instead of being so surprised by them.