I suspect some of the "thinness" will be mostly a visual trick, like with the iPad 2. That's not to say it won't actually be thinner, only that I think it's more about aesthetics than that they are chasing millimeters.
Yes, but we don't know how they do it.
All Apple products were always bigger inside than the outside dimensions.
....but seriously folks who cares?
If you sit in front of it you don't see the sides/depth. Only benefit I can see if they take out with the superdrive (outcry) and thinner saves weight.
Very important since everybody carries around their iMacs.