As mentioned, you technically have to have a Virgin Mobile phone. There are people who have claimed to have moved Sprint phones to VM (typically through ESN spoofing) but the legality of that is questionable.
Being able to use an unlocked phone is an advantage that GSM carriers have over the CDMA carriers. The reason for this is that GSM phones use a SIM card to store provider and account information (it is independent of the phone), on CDMA phones that information has to be programmed into the phone. Some people on a GSM carrier actually use multiple phones (though one at a time), just moving the SIM card to the phone they want to use that day.