It's not the frequencies, as Sprint 3G service DOES use PCS frequencies that verizon uses as I have an unlocked 3G modem in my computer that can use either Sprint or Verizon (AT&T & T-Mobile too) but the main issue here is ESN locking. I doubt that Verizon would accept a Sprint locked ESN on it's network. There are firmware flashing issues, PRL lists and other things too.
But the short of it is, that in this country, carriers do not sanction the interoperability of phones on each other's networks. Big difference to the way it is in Europe, where the network is standard and network sharing is common. Phone sales there can be made independent of carrier and interoperable by simply changing SIM's.