I've seen this story (NASDAQ being hacked) reported in a couple of places, but it isn't clear to me what damage was done. It's not really possible for them to have messed with the actual trading without anyone noticing. Everyone connecting to an exchange is reconciling the orders they send in against the trade confirmations they receive. You basically design your technology assuming the exchange is going to fuck some…
They could have hacked a customer (say, citigroup) and entered that way, but all they really could do is incur losses for the customer.