Yeah, I remember that..
I think they're talking about a health bar right above/below the monster though, like some other MMORPG.

Oh nevermind, I found it:
http://www.diablo3demo.com/gameplay-basics.html

The floating health bar is what they have been doing in the demo, and people don't quite like it so they said they're changing it to bottom stationary for normal mob, and top stationary for boss.