Another exchange that will offer stop loss orders (and many other order types) is kraken.com. Kraken is in open beta right now, so if you want to play around with stop loss orders to get the hang of it, you can sign up for an account here
beta.kraken.com
Your account will be funded with play money so you can try it out. The way trailing stop loss orders work is that you set a "stop offset" (either in absolute price, or a percentage) and a market sell order will trigger once price falls by the stop offset amount. For example, if you set a stop offset of 5%, the market sell won't trigger until price falls by 5%. Kraken should go live within two weeks, possibly next week. Note that your beta account will be wiped at launch, so you would have to open a new account for real trading.
Disclosure: I work for kraken.com (Payward), as vouched for here
https://bitcointalk.org/index.php?topic=192104.0