From my experience, you can use any GSM prepaid carrier by simply popping in an activated SIM Card.
To get on a CDMA prepaid carrier its tricky. There is nothing to 'flash' on your phone. You still need a SIM card to tell your phone what CDMA carrier to try and connect to. That CDMA carrier has to let you on there network. That process involves 'clearing' the network on your phone while the new sim is in and then talking to that carrier so they can add your IMEI to a whitelist.

What makes a 'Verizon' iPhone is that it was activated on there network at birth and its on there whitelist. It is not 'flashed' any different than other CDMA phone. It has just been given the blessing by Verizon to be used on there network and they will not activate any iPhone that was not previously on their network before. This is the policy of just about all CDMA carriers.

Basically GSM Carrier is a 1 step process. CDMA is a 2 step process and all the major carriers are dicks.

For example, all carriers unlock iPhone 4S's after contract expect for Sprint so you are still tied to there CDMA network even after your contract is up. However they will let you use an Sprint MVNOs.