Healing in Cataclysm will eat mana, loaaaats of it, period.
What a tank can do is learn the encounters, learn what mobs do (including trash) and act accordingly. A tank can do a lot to prevent damage either on himself and/or the group.
* some mobs need to be turned away from the group
* some mobs channel some crap onto you or the area around them, that can be avoided by simply moving away from them - a tank doesn't have to stick to the target like glue - move
* interrupt casts if possible
* stun
* slow
* kite
And possibly some more I can't think of atm. It just takes some getting used to. It's quite different from anything I've seen in WotLK and feels a lot more like early BC heroic runs (pre nerf in blue gear).