Nextel iDEN phones will work with Boost, no modifications necessary. You just insert the Boost SIM and go.

Boost iDEN phones, on the other hand, will not work with Nextel unless unlocked.

Please note that only the Motorola pure-iDEN phones--Boost or Nextel--will do the free data tethering trick. That means you're looking for Motorola phones with i### names. iDEN Blackberries and even non-i### Motorola phones will not give free tethering.