New spambot behavior. Sub-second buys and sells at prices only ticks apart. (micro-orders as well).
Question for Mt Gox - why aren't the people who own these banned for long periods or completely?
Because they're using 1000 accounts to do this - botnets spread all over the world. It's exceedingly difficult to match the owner to the behavior. The only way to eliminate it is to make it very expensive to do this.