How does the raid lockout system actually work in Cataclysm?

I understand you can now run both hc and normal, but in regards to the lockout itself, are you locked per boss or to the entire raid when you kill a boss?

So is it like retail, where you are locked based on the bosses killed, so you can only recieve loot from them once, or are you locked to the entire raid once you've started it, just like it already is in classic?