Helping a friend to unlock and jailbreak iPhone

My friend is trying to decide which iPhone to buy to unlock to use on T-mobile network. I heard that after jailbreaking, it is not possible to unlock anymore, is it true? I told her to buy a 3GS so that she has a better chance to get an iPhone with an iOS other than 5.0 and a processor other than A5. Did I tell her wrong? Thanks for replying.