I was so lucky, I registered on bitstamp on the 3rd of Jan and this hack happened just after.
Happy that I did not get involved in that site any further.
Most likely, it will, or already has happened, to all of them at some point. It's how they handle it when it happens that seperates the pros from the script kiddies. Remember, the underlying systems that run the exchanges are extremely complicated and there is always going to be a security hole somewhere, code, inside job or human error, it's there waiting and they are all constantly under attack.
Happens to MC and VISA multiple times a day but no mention is made unless it's huge. Same with banks. A certain amount of funds are set aside to deal with fraud and loss from theft but you don't hear about it unless it's massive. Let's see how Bitstamp comes out of this and what their final explanation is for the loss before we throw them under the bus...