Is is just me that thinks this is that totally unacceptable, for this site is one of the only Bitcoin exchanges in the US. This alone could cause panic trades when they eventually do get back up and running.
Anyone else is free to open up a competing exchange but it's such uncharted territory (mainly legally) that the risk outweighs the reward.
Bitcoins are in their infancy, remember. How they will ultimately be used, exchanged and valued has yet to be determined, and early adapters on all fronts are just going to have to deal with these things.
Edit: Just to add to this, during the last two attacks the guy that runs mtgox was in the bitcoin IRC channel working on the problem and giving people updates in real time. Both were fixed in a relatively timely manner (it's back up now). You really just can't ask for more from such a small operation.
I wonder if this could be a motive? Can you short sell bitcoins?