Bitcoins are completely durable. A fully anticipated decrease in supply of a durable good cannot cause price to increase any faster than the real interest rate (1-3% per annum). If not, then speculators would want to invest in the durable good now, store it, and sell it at a profit after the supply drop. Competition among speculators drives the expected price increase down to the real interest rate. Any price effect due to falling block reward is already loaded into the current market price.
If the supply decrease was unanticipated or bitcoins were not durable, then this would be a different story.
It's not this simple, for one simple reason. Bitcoin market is so volatile for a large number of reasons that you can't look at this in a vacuum. If speculator x thinks the price might go down in the near future for a reason not related to this, he is not going to be buying now. I don't believe that this particular event is well priced in at this point. It will become priced in once this event is a bigger issue in people's minds. Right now it's still far away, so far in Bitcoin time that for all we know we might have a super bubble and a bust before it even happens.
To hammer my point in clearer, it's easy to anticipate a resulting price increase because of the reward drop. But from where? From where it was before. Here is the problem, we don't really know if a price of $5 is high or low. Maybe we're at $3 in August, good luck for those who priced in the price increase now. Bitcoin is so full of questions that it would be sort of super bullish to price in the reward drop price increase at this point because it's not certain we're even this high in the months before the drop.
So I'd say that people buying now are buying for a wide variety of reasons, this one in particular having little effect. In the months before the event, many will be buying solely for this reason. That is the difference. It does add to the overall bullishness of already bullish market players but closer to the event this alone will cause significant rallies because people expect that. It's a self-fulfilling prophecy, just like everything else in an environment like this.