Yeah, noon to 3 pm is pretty much the witching hour for most wireless networks, not just AT&T. It's lunch time, and while most people have to sparingly and illicitly check their phones for personal email/twitter/facebook updates during work ours, they can openly do it during their lunch break.
And they are probably doing some sort of network management when usage is high, because they'd rather the speed of individual connections be a bit slower, to a bunch of people complaining that they can't get on at all. Which would you prefer?
As an aside: this is yet another reason I feel it was a bad move to change the 4S' functionality so that the user can't manually turn off 3G and drop to EDGE. Back in the days when AT&T's 3G was at its worst in NYC and California, you could at least drop to EDGE and get a slow-but-usable signal.