From the mtgox announcement
http://www.bitcoinrumors.com/2013/04/04/mtgox-announcement-on-ddos-attack/"Why has Mt.Gox become the target of a DDoS attack?
..
Abuse the system for profit.
Attackers wait until the price of Bitcoins reaches a certain value, sell, destabilize the exchange, wait for everybody to panic-sell their Bitcoins, wait for the price to drop to a certain amount, then stop the attack and start buying as much as they can. Repeat this two or three times like we saw over the past few days and they profit."
I speculate that the attackers are targeting vircurex and btc-e for the same reason. They would have placed low bids, attacked, then when the attacks stop there will be panic selling, their bids will scoop up the coins cheap, when prices return to normal in a few days they'll sell making a profit. They'll then use part of their profit to attack again in a while, and keep doing this as long as they make a profit.
To neutralize this, as soon as the exchanges reopen people can cancel their asks and place bids. This will slow the selling, and so reduce the attackers' profit. Without a profit they won't do this again. I pledge the following, for three days after the exchanges reopen I will:
1) Cancel my asks.
2) Place bids with my bitcoins already on the exchange.
3) Send in more bitcoins from my wallet, and place more bids.