If supply is cut in half and demand remains the same, market goes parabolic and you end up with $10,000 xboxs on ebay.  While we are not cutting the supply in half of all Bitcoin's in circulation, the price of Bitcoin is determined by float, which newly mined coins make up a huge part of.  

To emphasize the point, it's estimated by some that up to 30% of Bitcoins might have been lost by users in the early stages of the coin (but not nearly as many people lose coins now).  If you could prove or disprove this fact, it probably wouldn't even have an effect on price because it's just not part of the float of coins on the market.  So, while cutting non-float coins in half can even give you no effect on the market, cutting newly mined coins in half is where the big effect happens.