Good question, I have wondered this myself.
Looking at MBTrading, they have TTO, Threshold Trigger orders. Looks like it will do what you want, but I haven't executed one yet. Perhaps someone has and can confirm?
Threshold Triggered Order (TTO)
Specify two prices, an upper and lower price threshold. Once the inside market trades at either threshold, a market order is sent to the marketplace. TTOs were designed to help limit potential losses and lock-in potential profits.
http://www.mbtrading.com/OrderTypes.asp
Looking at MBTrading, they have TTO, Threshold Trigger orders. Looks like it will do what you want, but I haven't executed one yet. Perhaps someone has and can confirm?
Threshold Triggered Order (TTO)
Specify two prices, an upper and lower price threshold. Once the inside market trades at either threshold, a market order is sent to the marketplace. TTOs were designed to help limit potential losses and lock-in potential profits.
http://www.mbtrading.com/OrderTypes.asp