At this point (since the question was asked) the iphone 7 has now been released. If I remember right, when checking the iphone upgrade program prior to the release of 7, you could obtain the 6S actually just unlocked if you went to an apple store. It seems like that option is gone now that the 7 has been released.
Also, it looks like if you sign up for AT&T or T-mobile, the phone does not have CDMA enabled, but GSM is enabled if you get the phone for Verizon or Sprint.
Over the years, I've had no end of problems with phones that were "unlocked" but associated with a particular company when purchased variably when traveling to Europe, Asia, and South America. There does not seem to be any way to predict what will happen when you put a foreign SIM card in your device if it has been associated with an american company at some point, even if it is "unlocked".
Call me crazy, but I just want an actually unlocked, association-free phone through the upgrade program and be able to throw in a straighttalk sim, and then throw in a sim from whatever country I'm in later, i.e. how it's supposed to work. Considering that the upgrade program is a financed version of paying in full, is this possible in some way? (dropping verizon is fine... but then will it be locked in some obscure way when I am in Korea or something)? Not one person that I have ever spoken with at at&t, apple, straighttalk, verizon, etc over the years seems to fully understand the repurcussions of any level of locking once you leave this country...
Thanks