As far as I know you still can't (easily) activate a Sprint CDMA phone on Verizon's CDMA network.
I read somewhere that Verizon will activate "foreign" phones in-store. Next time I go to the US I planned to take my old phone (specs) with me and activate it with Verizon to avoid a large roaming bill but now I'm not sure whether that's actually possible or not. Does anyone know for sure?