Are all iPhone 3GS's unlock-able right now?

i'm buying used 3gs , know if baseband mattered (to unlock international use)? understand os version can downgraded i'm not sure former, or for. advice or help!

 

the iphone 3gs on apple store can either at&t or unlocked have 8gb model.

http://store.apple.com/us/browse/home/shop_iphone/family/iphone/iphone3gs

 

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