According to Steve Kondik, whom you might be familiar with if you happen to dabble with CyanogenMod, he confirmed that the AT&T version of the recently available Samsung Galaxy S4 would arrive locked right out of the box. Of course, what does that mean for the ordinary user? Basically, it translates to those developers having to take a whole lot more time before they can roll out custom ROMs on the handset. Still, this locked bootloader is definitely not unbreakable by any means, and it is just a matter of time before it is cracked.
According to Steve, “Yep, it’s confirmed. The AT&T S4 authenticates the recovery and boot images before executing them. I can’t see what AT&T has to possibly gain from this. GSM and LTE aren’t magical, tethering is controllable on the server side, and theft-of-services is not possible from the application processor side (or even from the modem side as far as I know). The same device is available on every carrier, so it’s not an exclusivity issue either. The modem processor has always been locked, and the casual user doesn’t want to mess with that part anyway. Samsung has always been developer-friendly, so I am guessing their hand was forced.”