AT&T probably won't unlock it for you, but unlocking the HTC One is ~$3.50 on eBay. No problem.
Be careful though and you might want to read the slickdeals thread posted in the OP as people have been getting inconsistent experiences when they're trying to cancel. Canceling within 3 days is legit, and it's in the rules, but you might have to fight a little bit if you're unlucky enough to get a clueless (or overzealous) CS person (in store or on the phone). So just be aware that there might be a little hassle coming with it (or not).
I think it is safe to assume that by tomorrow all of the CS reps will be aware of this loophole. Order today with overnight shipping if you still want to try it.
Also I unlocked my GS4 in 5 minutes using a few quick steps on XDA devs in the debug menu. Not sure if the One has such an easy unlock though.
