Let's say the block reward change occurs on December 9th, 2012. On December 8th there are 10,492,800 BTC issued and another 7,200 will be issued that day. The "supply' comes from those 7,200 that miners don't keep plus a certain amount of the 10,492.800 BTC that those holding them are wishing to sell at whatever the current price is.
So demand does not drop in half, only the amount of currency inflation is what drops by half (from 7,200 BTC to 3,600 BTC).
Incidentally, in a market where the commodity is scarce, a doubling in demand causes the price to rise by way more than double.
You are forgetting the effect this news has on people
Just like SoiledCon when King RealScam announced the change to cut reward the price doubled in the same day as people bought up the coins !
But it's very easy to double the price of something worth in the tenths or hundredths of a cent...
I see the way they do it over at BTC-e. Pretty much a daily routine on whichever flavor it happened to be that week.
Good point, plus announcing you are changing it is very different from the BTC situation. It is well known and will be mostly priced in by the time we get there.