Slight correction: there's no such thing as an unlocked USA CDMA phone. In other countries (Japan, China and I think a few in Europe) there are CDMA networks that allow unlocked phones.
This may just be a matter of semantics, but by its very nature a CDMA phone can't be locked or unlocked in the way that GSM phones are. When a GSM phone is "locked" to a carrier it means that only that carrier's SIM cards will operate with the phone. Unlocked means that any carrier's SIM card will work in the phone.
With GSM SIM cards, once the card is active, you can put it in any GSM phone and use it on that carrier... or put a SIM in the phone from any other GSM carrier and use the phone on their network. That is the nature of "unlocked" GSM. There is no need to get permission from a carrier to use a particular phone on their network. All the access to the network resides with the SIM, not the phone.
Since CDMA phones (for the most part, LTE aside) don't use SIM cards, it isn't the phone that is locked or unlocked to the carrier. The control of whether or not a particular CDMA phone will work on a carrier rests with the carrier. That's why you have to have a CDMA phone "activated" by a CDMA carrier. Put another way, even if Sprint were willing to release one of their EVO's serial numbers to be used on another network, Verizon is under no obligation to allow it on their network. Verizon has to allow the phone to work by adding its serial number to their database.
So having an "unlocked" CDMA phone is really analogous to having a phone that no carrier owns, because its serial number wouldn't be in anyone's database. Since CDMA carriers will only activate phones whose serial numbers they own, you wouldn't be able to just hop onto a carrier with that phone.
Non-U.S. CDMA carriers may be more open to exchanging serial numbers in order to activate another carrier's phone, but again, it's not really an "unlocked" phone since you have to get permission to use the phone on said network.
More information as to why the concept of an "unlocked' CDMA phone is completely different from what people consider unlocked with GSM:
http://theunlockr.com/2010/09/15/unlocking-101-difference-between-cdma-and-gsm/