When news broke that Verizon's and AT&T's versions of the Galaxy S4 would ship with locked/non-unlockable bootloaders, people were... upset. This sort of action was basically expected from Verizon, but AT&T had historically left its device's bootloaders unlocked, allowing users to do what they wanted with their own handsets. To make matters worse, the Galaxy S4's bootloader signature verification is nearly impossible to crack.
Then, at the first of the month, all-around genius hacker Dan Rosenberg released a teaser for his upcoming tool that would "hack" the AT&T's versions bootloader. That tool, now knows as Loki, is finally available for download, and it also works on Verizon's version of the device.
Before you get too excited, though, it's worth noting that this is a developer-only tool. It's designed for ROM and recovery devs and is not a utility made for the average Joe. In other words, don't expect to just download Loki and flash it through ADB or some other method.
The good news here is that it has been done. This tool, once implemented, will allow the bootloader to be bypassed and a custom kernel to be executed. According to Dan, this is similar to kexec "in that it works around a locked bootloader, but this approach is much more flexible and robust."