http://www.wowhead.com/blog=172336
I've just been reading Wowhead's summary of Blizcon.

One thing that really struck out to me was that Blizzard have said that ideally they'd like raids to be able to be completed in one day.

I'm guessing that's not going to be possible for new raids/progress runs.
I don't see us coming to a new raid completely fresh, and downing all of the bosses within 3 hours ^_^;;

But I'm guessing it might have an implication when we get a raid on farm. They might all start looking like Gruul's lair in terms of we turn up and do them in one sitting.

Yeah, they have been saying for a while they would prefer to have a couple of 6-boss raids per tier rather than a single 12-boss raid. It's a good thing IMO - the new lockout system is good, but it is still quite strict with heroic modes, having 2 separate raids will give more flexibility when it comes to heroic lockouts.