I saw your other post, so you already know that it's not carried by Cingular, yet. Word now is that it may come out through Cingular by Oct. But then that's just what it is. I think what what bubbletea meant is that when a cell phone is made from the manufacturer it is essentially unlocked. When a provider decides to carry a certain cell phone it becomes locked to that specific provider. So when you hear people talking about unlocking a phone, it just means you are lifting the restrictions that were placed from the carrier/provider. So now you can use the phone on any GSM network.
Just remember that when you purchase an unlocked phone you have to put in the settings for mobile web and MMS messaging.