* : The iPhone 4 connects via 3G for your voice and data needs (at least when there's no Wi-Fi network available, in the case of data). But that faster connectivity won't be available to you if you were to, say, latch on with T-Mobile's network. As jailbreakers who unlock their iPhones against Apple's wishes already know, T-Mobile uses a different 3G frequency than AT&T does; consequently, an unlocked iPhone would only be able to connect to the slower EDGE network for data. On the bright side, voice features should work fine, though since Visual Voicemail is a carrier-supported feature, you wouldn't have access to that, either.

Like many decisions involving iOS devices--AT&T or Verizon for your service provider? 3G or Wi-Fi-only for your iPad?--whether or not to buy an unlocked iPhone comes down to your preferences and how you plan on using the device. For users who can't stand the thought of being bound to one carrier for two years, $649 to $749 will seem like a small price to pay. Travelers who hop from one end of the globe to another--particularly on business--might also find the flexibility of an unlocked iPhone to be worth the price. Most other people will prefer the low initial cost of an iPhone to a month-to-month agreement with a carrier.

Whatever you decide, you'll have to do it at the time of initial purchase. You can't unlock an iPhone after you've already agreed to a two-year contract with a carrier--buying your phone unlocked at the start is the only option you have.