I'm pretty sure you won't be able to use on Nextel's network, unless you know someone that has already done it.

Even though both AT&T and Nextel use SIM cards, their networks/towers are different. Nextel runs off iDEN and AT&T uses GSM, so technically the phone shouldn't work, even if it was unlocked, because when it's unlocked, it's unlocked only to other GSM networks