I am really getting sick of seeing sellers claiming that the CDMA phones they are selling are unlocked. Now while most GSM phones can be unlocked, CDMA phones are another story.
GSM carriers include Cingular and T-Mobile; CDMA carriers include Sprint, Verizon, and Alltel. There are way too many to include in this list, so I just included the major American carriers.

What to understand. GSM is a mobile standard. It is general referred to as the European, because it started in Europe. GSM is the network configuration that your phone will operate on when using that carrier. GSM phones are more simple then CDMA. Apart from a carrier specific splash screen and sometimes a custom menu setup, they are rather basic. This allows for them to be unlocked for use on other carrier's networks. As long as they operate in a Frequency used in that country. In the USA 800Mhz and 1900Mhz are the norm. Pretty much all GSM phones use a software interface called Java. Keep this in mind because this will play into why a CDMA phone can not be unlocked and used on another carriers network.

CDMA is an American standard used in many places around the world. It is the most commonly used network standard in the USA. CDMA phones by nature are far more complicated. They use specially designed custom software that is put in place for a set carrier. This makes it next to impossible for someone to unlock it. Also, CDMA phones use a serial code called an ESN. This is used to make sure the phone stays with that carrier. It also allows them to lock the phone if it is stolen. Every CDMA carrier has a list of ESN's, for every phone being used on their network. So Sprint has their own, Verizon has their own, ect. If you were to take a Verizon phone and try and activate it on Sprint's network, the ESN wouldn't match and they would not allow it.
Also, Sprint and Verizon use a different User Interface. Sprint uses Java and Verizon uses BREW. These are two completely different software standards. Compare them to Microsoft and Lunix. Microsoft having their source code locked, while Lunix is open source. Basically this means you can use all kinds of free Java software on your phone, while there is no free BREW applications.
This is important because even if you found a way of getting Verizon to activate your Sprint phone on their network, it would not work at all. The software is just completely different. It should also be pointed out that Sprint is the only CDMA carrier to use Java. Their MVNO's like Virgin Mobile and Helio, who use Sprint's network, also have their phones locked. So even though Virgin and Sprint are using the very same network, you can't use one of their phones under the others brand. This was also the case for Amp'd who was using Verizon's network.

And just for reference, No, you can not use a Nextel phone on either a GSM or CDMA network.  While you can buy an unlocked Nextel (iDEN) phone, it will only allow you to use Boost Mobile or Southern Link phones on their network. Which is pointless because they are all the same Motorola phones.

So keep this in mind when when you see sellers claiming that they offer unlocked CDMA phones. They are either unaware of the differences, or just lying to con you into buying their phones. An unlocked CDMA phone is a scam. You can't use it on any other network. Maybe else where in the world, but not here in the USA. Hopefully one day this will be the case, but as of today, CDMA phones can not be unlocked.