CDMA iPhones (and CDMA phones in general) are not unlockable (in the sense that you can go from one carrier to another on a whim), period.
If you're getting a GSM phone, there is a very good probability that it will come with 4.3.4 (either now or within a very few days when they get them all updated) which is not jb/unlock compatible at this time.
Other options would be to buy one that's factory unlocked (outside the US mostly, though you might be able to get one here as well), buy a used one that has 4.3.3 or lower on it (I think), or wait until the new jb/unlock comes around.