I could have written a much longer description but I think that the Ars Technica article covers it well. The claim that Mt. Gox is the only other exchange is silly though. There are many exchanges and Tradehill was only slightly bigger than Intersango and Cryptoxchange. Both of these exchanges have started to push their marketing especially at Bitcointalk forums, after this incident: https://bitcointalk.org/index.php?topic=63941.0
So that everyone clearly understands, Mt. Gox is by far the biggest exchange. Even though Tradehill was the 2nd biggest exchange, it was tiny compared to Mt. Gox. Both Intersango and Cryptoxchange were almost as big as Tradehill at the time of its demise. This information is available at http://bitcoincharts.com/markets/
I didn't pull anything out of my hat but I could have perhaps added a few more links and a slightly longer description.