Buying lots of bitcoins with dollars, driving the price up (and maybe creating an expectations bubble), then waiting for the price to stabilize before selling all you've bought (hoping to create panic selling) would, in my humble opinion, be an effective attack by anybody who had a lot of money to spend, didn't mind making the owners of the bitcoin exchanges very wealthy, and wanted to create fear, uncertainty, and doubt in the bitcoin market.
Of course, if there was somebody with a lot of bitcoins willing to sell on the way up and buy on the way down to stabilize prices...