I sold my two unlocked 640's, Lumia Icon, and Note 4 last week as I had way too many phones and way too little cash. But then I missed playing with my 640 so I went ahead and got another one at best buy. I can see at&t is clearly still giving a hard time with unlocking as I received another NUL request code, and after 4 days "still processing" my request. I am 95% sure I will get another approval email with a blank unlock code if I even get one at all.
EDIT: Just received an email from AT&T that my request to unlock my device was denied because I didn't respond to the conformation email. This is false. Just another blatant tactic AT&T is employing to skirt it's own unlocking policy. Again, there is no bug in the system and it doesn't cost AT&T a dime to throw a wrench in the unlocking process.