Actually, another interesting factoid I ran across while doing some reading--apparently concrete does not truly finish curing for a loooooong time. The concrete in the Hoover dam is still curing on the inside (I remember hearing that on an episode of Modern Marvels or some show like that on the History Channel). There are very old concrete structures that are still curing. One benefit of this slow curing is concrete's ability to slowly self-seal small cracks over time, which wouldn't be possible if concrete were totally hardened and static.

Obviously, in applications where you absolutely want the concrete cured all the way through, I'm sure that there are formulations or methods that allow that.