How can I use an iPhone on the TracFone network?

You should be aware that changing the SIM card is not a solution because when you go to activate, Tracfone will ask for the IMEI of the phone and this is a unique to each phone. They have a list of those numbers associated with all the phones they typically sell to the public and the one on your iPhone is not on their list so you cannot activate the service. You should also be advised that changing the IMEI number is a felony. Basically you are not going to meet with success and it is a pity since the Tracfone service is great but the stock phones that go with it are outdated, underpowered and not what many people are willing to settle for.

TracFone, per the eHow guide, relies on a SIM card to work. The CDMA standard does not commonly use SIM cards, especially not in America. TracFone also works on pre-iPhone 4 iPhones.

The conclusion?
Get any version of the iPhone, but get a GSM version if a choice needs to be made.

The next question is, does TracFone provide a micro-SIM for use in an iPhone 4 / 4S?