Sprint will only ever activate a phone who's electronic serial number (ESN) originated from their system. They will never ever add an ESN for a device that wasn't sold for their network, even if the device is from the network of a MVNO that uses Sprint, like Virgin. Anyone who says differently is either lying or did some funny business with ESN cloning/swapping. Verizon is the same.
Some people, myself included, see that as one of the pluses of GSM networks .... no ESN's .... all sim card. Unlocked phone + sim = GSM phone will work on any GSM network as long as the physical radio supports the proper bands.
With that said, sprints prices and killer phone selection topped off by my preference for CDMA technology (shorter travel but higher quality and better building penetration) are enough to keep me from worrying about stuff like sim cards and GSM networks.