Just to add, Nextel (and Boost, since it is the same company) are not GSM, and will not accept GSM phones, nor will their SIMs work in GSM phones. They use a different frequency band and a technology called iDEN. Newer models may also support CDMA as well.
Also, US carriers do not usually supply unlock codes for any reason. It seems they like to have piles of useless phones in landfills.
ATT and T Mobile will accept unlocked GSM phones for use in the networks, but you have to make sure that the phone is quad band, as the european dual band phones do not have the right bands for US networks. T mobile will give you any contract plans without needing a contract if you bring your own phone, but then they refuse to insure the handset.