First of all, stock, not stack exchange.
I just read this post on Krypt3ia. In short, it says that HFT attack vector in combination with DDoS is possible new nemesis for stock exchange IT systems. Later on in article author describes how poor IT security is on stocks.
Anyway, as far as I know HFT is... Hmm, metodology? Technology? Well, "something" which demands extremly low latency, very accurate (and very fast) forecasting, being able to process millions of transactions if needed... All in all cutting-edge algorithms, technology and so on.
Question is - how HFT could be used as attack vector? How would that attack looks like? And, of course, could this kind of attack shut down stock exchange?