Same basic problem with a new MacBook Air (mid 2013) 8GB/256GB.
Symptoms are a little different, but this has happened to me three times over the past two weeks. I think it was running on battery each time, but not 100% sure about the first time.
I walk away for a few minutes to refill a coffee cup, or take a phone call. When I come back the screen is still on so I move the mouse to start working. Before pressing any keys the computer is clearly awake and the mouse is visibly moving so it's jarring when the screen goes black. At that point, moving the mouse has no affect, but pressing any key wakes the screen and everything is normal.
I can understand that it's possible that I sat back down and moved the mouse at exactly the same time the machine was going to sleep, but it's unlikely that could happen three times in such a short span.
Before this Mac I had a late 2010 Air with all of the latest software updates, so I am very familiar with this operating system. Never saw this behavior with my old Mac. No other obvious problems/symptoms with the new Mac.
It feels like a software problem with the sleep timeout.