Do any of the Bitcoin exchanges implement trading curbs?
Mt. Gox has stated that they now use "circuit breakers" to disable trading pending manual intervention if trading appears suspicious or unreasonable. They reported a few minor incidents where the breakers did trip in the initial implementation due to unusually high, but normal and innocuous, volatility.

My recollection is that TradeHill only has breakers on the movement of Bitcoins out of the exchange. So far as I know, they have no breakers on trading -- at least not that they've announced.