Would you be willing to place a 0.5 BTC bet?
If the price goes up during the 24 hour period after Amazon announces they are accepting bitcoin, you give me 0.5 BTC.
If the price goes down or remains exactly unchanged during the 24 hour period after Amazon announces they are accepting bitcoin, I give you 0.5 BTC.
This is a terrible bet because btc price most likely has zero correlation to whether Amazon accepts btc payments or not. During this 24hour period, what if Satoshi decides to dump his btc's? Or what if Warren Buffett decides to buy up the whole market? Examples are a bit extreme, but you get the point. Or, when Amazon announces btc payment option, it could just simply be normal market fluctuations, nothing to do with the announcement.