Well, there had to be one that spoiled all the fun! Verizon is the sole carrier that offers Samsung Galaxy S III devices with boot loaders locked down. Sure they will have a “Developer Edition” phone but its announcement should have gone live before people started pre-ordering and purchasing the “regular” (locked) device.
However, the community is working hard on trying to bypass the boot loader lock on the Verizon Galaxy S III. After just recently obtaining root, devs are working on what they call a Kexec hard boot bypass of the boot loader (“a method of booting kernels through recovery without needing to flash them to the device”). This would allow users to flash custom ROMs despite the boot loader being locked down. This is, currently, a proof of concept and is intended for “developers and experienced testers”. If you want to stay away from that, you might want to go the “Developer Edition” way; else, hit up the source link to read more.