If you’ve already pre-ordered your iPhone 4S, chances are you’ve noticed that the option to buy an unlocked 4S in the US won’t be available until November.

A specific date has not been given, but US iPhone customers will have to wait for a carrier unlocked 4S. Customers in the UK can buy pre-order an unlocked 4S today.

“If you choose AT&T as your wireless carrier, your iPhone 4 or iPhone 4S will use only GSM cellular technology for voice and data services, and it can roam internationally on 200 GSM carriers around the world.

If you choose Sprint or Verizon Wireless as your wireless carrier, your iPhone 4 will use only CDMA cellular technology for voice and data services. An iPhone 4S activated on Sprint or Verizon Wireless will use CDMA technology too, but it also has the ability to roam internationally on GSM networks using the provided GSM micro-SIM.”

The iPhone 4S is a “world phone” capable of running on both GSM and CDMA carriers, but buying a 4S for a specific network seems to lock you to that baseband. This means that AT&T 4S customers won’t be able to just switch over to Verizon or Sprint at the drop of a hat.

AT&T customers should be able to remove their microSIM and use T-Mobile’s 3G network on an unlocked, GSM-activated iPhone 4S.

Check out our top 10 reasons to buy the iPhone 4S roundup for more information on this sweet phone.