Boost also just like nextel is hybrid for "cdma" and "iDen" :
http://en.wikipedia.org/wiki/Boost_Mobile
"United States
As Boost Mobile is wholly owned by Sprint Nextel they run on both Sprint's CDMA network and Nextel's iDEN network. Unlimited by Boost is designated for the CDMA handsets, while PayGo is for the iDEN side."
as stated on the wiki page also most Nextel phones are compatible with boost mobile cards...
Did a little research and was considering the move...Or just another line!!!