I don't really know much about PS3 hacking, so this is all just a guess.
This is how original Xbox and Xbox 360 softmods work. The regular OS boots and either an exploit occurs in the font package, music player, MechAssault or Splinter Cell (the latter two used for bootstrapping the former two) or in the 360 the xbox is "rebooted" virtually to avoid the verified boot into an alternative kernel that has signature checking removed.
Note, I know nothing about PS3 hacking either and am making assumptions based on the connotations of the word "master key", "signing" and other comments here along with my knowledge of xbox1/360 hacking.
It's not the master key for cryptographically signing executables or OS images.
http://www.youtube.com/watch?v=X6CA4fqAdsc
I'll post more info once I've seen what they say.
EDIT: WOW! Okay, looks like they screwed up big time. They used the same random number for all their signatures, which means that they effectively leaked their private key for various bootloaders in the system. The chain of trust is toast.
The master keys for signing PS3 executables has not been leaked.