This has been happening to my rMBP since Dev preview 3. When in Safari and always on battery power, the system will freeze. This happens very rarely, even in the GM. I cleaned up my system, launch agents, daemons, etc, and since then it hasn't frozen again. That doesn't mean that the problem is solved though. I guess we will have to see..
I have submitted a bug report a while ago but Apple couldn't find the reason for the freezes.