Not that much, actually. If I'm reading it right, you could sell 50K BTC at $3.90, causing the market to drop "only" 17%. http://bitcoincharts.com/markets/mtgoxUSD_depth.html

And some selling is going on: http://bitcoincharts.com/charts/mtgoxUSD#rg1ztgSzm1g10zm2g25...

As someone who is unfamiliar with how exchanges work, I'm curious what an "ask" for 1.337 trillion dollars in bitcoins means, exactly:

  1337000000000.00 	1 (1) 	272981 	1405085033491

Not entirely sure, but I believe this represents someone willing to sell 1 BTC for $1,337,000,000,000.00, where all sell orders in the order book add up to 272981 BTC and USD$1,405,085,033,491

Using the current mtgox order book (mtgoxlive.com), such a sell would push the price from $4.60 (where it is now) down to about $4.00.

One of the hacked exchanges said they'd reimburse their clients, so I guess they'll buy roughly the amount that was stolen.