If you're in the middle of a contract, it's a costly proposition. If you purchased an iPhone with AT&T and want to leave the contract early, The same situation at Verizon results , minus $10 per completed month of contract. Leaving Sprint early--if you've purchased an "advanced device" as part of the contract--.

Because the iPhone 4S is a world phone--meaning a single model is compatible with both GSM and CDMA networks--one might assume that you'll be able to easily switch carriers at the end of your contract while keeping your current hardware. One would assume incorrectly: Apple's fine print on the iPhone 4S warns that "the iPhone model you purchase is subject to your wireless service provider's policies, which may include restrictions on switching service providers and roaming, even after conclusion of any required minimum service contract."

The list price of $199 for the basic 16 GB model is actually a "subsidized" price--it assumes you'll sign a new two-year contract with a wireless phone carrier. If you want to upgrade your iPhone before reaching upgrade eligibility, you can always pay the unsubsidized cost. And how much is that? Tough to say. Apple announced of the iPhone 4S at its Tuesday event. So it's worth noting that the unsubsidized cost of a 16 GB iPhone 4 was, at the time of its release, $599. If you want the new phone and you're not eligible for an upgrade, you'll probably have to pay a steep cost.