And so thought they on November 28th 2012 that they would wake up the next morning to a price increase.....nothing happened. The market will have this priced in long before the action.
I think it's more plausible this time, but still improbable. The impression I have of the mining market now is that everything's done on much tighter margins, and miners that aren't speculating ideally need to make at least some amount of coins to break even. When the reward halves, most miners conceivably will need twice the buying power per Bitcoin to keep their profits - or lack thereof.
It certainly will not be instantaneous, but I have a strong feeling that if miners are still on the edge when this halving date encroaches, prices will do something interesting.