Q: I just bought an iPhone 4G without a contract and I have a few questions. How do I activate it? How do I use it only as an iPod touch? How do I use it with a phone carrier like T-Mobile if I want to in the future? Will any of this void the warranty? I do not wish to jail break.
A: The iPhone 4 is not designed to be used without a SIM card or a cell phone plan of some kind. It must activated with iTunes before it can be used, and in order to do this iTunes needs to see a valid micro-SIM card in the device.
That said, iTunes only checks validity based on the ID of the SIM card—it doesn’t care whether it has an iPhone plan associated with it or even if it’s on an active account at all. Once activated, you can continue using the iPhone 4 without any of the cellular phone or data features much like an iPod touch. Note that FaceTime will not work on an iPhone 4 without an active cellular plan, however, as the iPhone 4 version of FaceTime uses a cellular phone number to register you with the FaceTime network.
Without a SIM card inserted there is no way to activate an iPhone without jailbreaking it.
Note that if you purchased an iPhone 4 in the U.S. even without a contract, your iPhone will still be locked to AT&T and you will therefore need to use an AT&T micro-SIM card in order to activate it, although again you do not need to have an active account on that SIM card—iTunes just needs to see it in the iPhone. Also keep in mind that because the iPhone will still be locked to AT&T you will not be able to use it with another carrier such as T-Mobile without unlocking it first, which requires jailbreaking it.
If you purchased a contract-free phone outside of the U.S. then it may be officially unlocked already in which case you can use a SIM card from any carrier in order to activate it, and you will be able to use it on any carrier you like. Keep in mind, however, that not all carriers support the iPhone 4—even GSM carriers such as T-Mobile in the U.S. use different frequencies for their 3G network that are not compatible with the iPhone. Using an iPhone 4 on T-Mobile will work fine for voice calls, but for data you will only be able to use the much slower EDGE network.
Short of actually jailbreaking your iPhone, none of this will void the warranty as you’re not modifying the iPhone in any way—merely activating it with iTunes like you’re supposed to.