Blackberry curve with an existing sim?

So I broke my blackjack and I don't want to get another contract from at&t so I'm considering just buying a new blackberry curve from Ebay or possibly Bestbuy, I'm not sure what the whole unlocking thing means but I just need one thing clarified, if i buy an "unlocked" blackberry 8520 curve off ebay can i use my existing 3G sim card from at&t?