I have a used 902 in my shop. VERY nice phone.
The SIM is married to the phone at activation, it does not require unlocking. However, and used phone would not work, because it would need to be partnered with the sim at activation. If you insert any other sim, Nextel or Boost, it will not turn on the radio, just like a subsidy lock on a GSM phone.
On any Powersource/Hybrid device the nextel network is ONLY used for Direct Connect. Not voice, Data, or SMS, even in a non sprint area.
So you have two obstacles to over come.
Sim provisioning, and Data provisioning. As we all know, sprint phones are very hard to provision for Data, let alone figuring out how it locks the phone to one ICCID.
Boost phones are STILL locked, and will ALWAYS be locked.
You can activate a NEW nextel sim in any boost phone, as lone as its an unprovisioned SIM. However the phone will relock itself in time, I have experienced this over and over again.
You have to unlock them. There are free programs out there, and a pay server that works flawlessly at IdengoRAB.org. I paid $75 for 50 unlock credits from them, not a bad deal.