In the first three years of the iPhone’s existence, the handset was crippled by limited availability. The only carrier in the US that offered the smartphone was AT&T. And in other countries, the device’s exclusivity was just as bad.

This year, however, Apple has been making some major strides. With the iPhone 4S, three out of the four major US carriers will have access to the device. But what does the new handset mean for T-Mobile customers and other unlockers?

Unlocking, for those who aren’t familiar, is the process of removing carrier restrictions on a phone to enable it to work with another operator. In the past, iPhones weren’t sold unlocked, so you had to go through a procedure to make it so.

Recently, though, Apple has been selling its smartphone completely unlocked. It’s a bit more pricey than purchasing one with a carrier subsidy, but users can easily switch the phone from AT&T, to another GSM carrier like T-Mobile.

It seems like the trend will continue, as MacTrast points to a description of the unlocked iPhone on Apple’s website:

“The unlocked iPhone includes all of the features of iPhone but without a contract commitment. You can activate and use it on the supported GSM wireless network of your choice, such as AT&T in the United States. The unlocked iPhone 4 or iPhone 4S will not work with CDMA-based carriers such as Verizon Wireless or Sprint.

If you don’t want a multiyear service contract or if you prefer to use a local carrier when traveling abroad, the unlocked iPhone is the best choice. It arrives without a micro-SIM card, so you’ll need an active micro-SIM card from any supported GSM carrier worldwide.”

It’s odd that Apple mentions that the unlocked iPhone 4S won’t work on CDMA networks, since the phone contains both CDMA and GSM support. It’ll be interesting to see if the jailbreak community can do something about that.

It sounds like T-Mobile users are in the clear, especially since the carrier supplies the required micro-SIM cards. Chances are that the new device will still be limited to 2G browsing, as it’s still not officially supported by Apple.

There doesn’t seem to be any pricing for the unlocked 4S on Apple’s website yet, but it should be available in the next few weeks. Plan on it mirroring its predecessors, with the base model starting close to $650.

Do you plan on buying an unlocked iPhone 4S?