For all of you out there who have been lusting after an iPhone 4S but didn't want to be chained year after year to one mobile carrier or another, rejoice! Well, actually, don't jump for joy unless you have a thick wallet to cushion the landing.

Apple (Nasdaq: AAPL) has announced that starting today you can buy an unlocked version of the iPhone 4S. The price, however, for forgoing a contract with Verizon (NYSE: VZ), AT&T (NYSE: T), or Sprint Nextel (NYSE: S) will add $450 -- at least -- to your bill.

Yes, an unlocked iPhone, unfettered by contract, will cost $649, $749, or $849, depending if you buy the 16GB, 32GB, or 64GB model, respectively. If you can afford that premium, it could be worth it. Frequent overseas travelers, for example, can change SIM (subscriber identity module) cards to take advantage of using a foreign carrier's GSM network instead of piling up huge roaming charges.

Oh, speaking of SIM cards, Sprint also made some news today. An internal Sprint memo -- one that may or may not have been leaked -- indicates that the company plans on locking the SIMs on its iPhones starting today.

So what does that mean? A SIM stores a user's international mobile subscriber identity and all the related information used to validate a particular handset. You can't make a mobile phone call without a SIM. If a SIM is locked, that means that SIM can be used only with that phone, and vice versa.

But this locked SIM news shouldn't be too surprising. When Sprint started selling its iPhones, they were not locked, but according to this press release from Sprint:

"Sprint's policy is to have the iPhone 4S SIM locked to our network domestically and internationally. Beginning on November 11, the International SIM will be locked for devices provisioned on the Sprint network."

But there is some relief in sight for those with seven-league boots. The press release goes on to say that after 90 days, if a customer is in good standing -- whatever that entails beyond paying one's bill -- that customer can request to have the SIM unlocked for international travel.

Keep an eye on what's happening with Sprint and Apple by putting them on My Watchlist:

Fool contributor Dan Radovsky owns shares of AT&T. The Motley Fool owns shares of Apple. Motley Fool newsletter services have recommended buying shares of and creating a bull call spread position in Apple. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.