There are a few developers that have been making a ton of progress on the Galaxy S III even though the bootloader is locked down tight. The bootloader situation has not really been an issue for developer "BeansTown106" who has developed on of the more popular Roms that is out now, his Bean's Custom Stock Rom. It is currently in its 3rd release and has seen lots of progress since its initial release. The Rom is Rooted and Deodexed, It is 4.0.4 ICS Stock. It includes all sorts of customizations and optimizations!
Some things that have been included are Wifi tethering, CPU script, Customized Toggles, Themed Toggles and battery, Center Clock flashable mod, Jelly Bean animations to give your phone that buttery smooth feel, Native call recording, MIUI/CM ringtones, AOSP keyboard, Build.prop tweaks for speed and stability, 4 way reboot menu for easy access to custom recovery and bootloader, bravia engine, beats audio integrated, removed carrier label, removed verizon bloat, 50GB of free dropbox storage and lots more.
I know that we already have the kexec boot process along with CM10 and AOKP, however those roms are still being perfected and Bean's Custom Stock Rom is based on Stock and seems to be one of the most stable and suitable for daily driver usage.
Grab this Rom here via Rootzwiki