NB: If you are avoiding spoilers, you can still read - I put a warning before any possible Cataclysm spoilers.
In ICC right now, I am, for the most part, the tank healer. I've always enjoyed tank healing as a Disc Priest. In ten man, this is a great setup, and on the rare occasions that I do run in 25 mans anymore, I am usually a shield spammer - which bores me to no end. I'm also very bored during the first Phase of the Lich King fight for that very same reason (but honestly, when you've done Phase one 19 billion times, I think everyone is bored at that point!).
One of the things Disc Priests were promised for Cataclysm was an increase in our ability to be solid tank healers. Will we be able to compete with the Holy Pally? Perhaps - it is still far too early to determine that for sure, but to me, one of the biggest reasons Holy Pallies are often better choices as tank healers has to do with their Beacon of Light, and the way boss fights were designed in Wrath.
Tank Healing in Wrath
Instead of the Main Tank/Off Tank setup in which the off tank rounds up adds and handles them while the MT focuses on the boss, we've had "tank swap fights". Every so often the tanks will have to swap due to some debuff given by the boss to the current tank. Sometimes (such as in the Toravon fight) this causes one tank to continue taking damage until the debuff wears off. Pallies are great for fights like this, because there isn't a single target to heal - these are actually dual target healing assignments, with one target needing more dynamic healing focus, and the other being safe with large "splash" heals from Beacon of Light. (more like giant tsunamis than splashes, but whatever)
As far as pure healing output, that is more of a numbers and throughput issue, one that can possibly be addressed in talents. Can Disc priests rise to the occasion with talent changes?
While our Power Word: Shield is great for shield-spamming, I've always felt it was a bit lackluster for tank healing. In end-game raiding, even the most powerful shields will often be blown away after just a few hits on the tank, and we are limited in our mitigation abilities by Weakened Soul. The main benefit from bubbling the tank comes not in the few hits that our bubble can mitigate before we can bubble again, but from the bonus healing we can do on them while they have Weakened Soul. We put a bubble on the tank, the tank has Weakened Soul, our heals have an increased chance to crit, and we therefore have a better chance of getting Divine Aegis procs on them, in addition to the stacking throughput bonus of Grace. The real mitigation comes from Divine Aegis. (Like all the math I used to back that statement up?)
Tank Healing in Cataclysm (talent spoilers)
Power Word: Shield will become more powerful and more useful in Cataclysm tank healing, because we will be able to cast it on the tanks more often. Here's a look at how the talents will change tank healing.
Renewed Hope will still give us an increased change to crit on the bubbled target - but instead of a 4% increase, we will have a 10% increase. Therefore, we will be critting more, and proccing more Divine Aegis mitigation.
Strength of Soul - This is the talent that will allow us to bubble the tank more often. Depending on your haste, you will be able to bubble the tank, and cast Heal on them until Weakened Soul falls off, and then Bubble them again, and continue forth. This means that Heal becomes a solid tank healing spell, so what follows are the talents that will affect Heal. Obviously, this is the most simplistic way to do it, since you will want to cast more than just Heal, but it gets the point across.
Grace procs from Heal, steadily increasing the throughput of your Heal spell.
I'm really enjoying the talent changes for Disc Priests for tank healing. What is particularly interesting about Strength of Soul is that it will give us a lot more power and flexibility in managing Weakened Soul. If you are specced into Improved Power Word: Shield, or have a lot of mastery rating (and therefore stronger bubbles), it is possible you can let that Weakened Soul sit there longer, as your bubbles will last longer. Instead of casting Heal, you can focus more on using your other healing abilities. Perhaps you can even start Smiting the boss and just getting some Atonement splashes on the tank and melee (the weeping of healers all over the internet is hurting my ears after that statement).
You can tailor your healing style even more to the encounter. For instance, in Phase 1 of the Lich King, he seems to be only tickling our tank, but once we get into Phase 2, he pulls out his mighty Soul Reaper. My tank healing rotation will change depending on the encounter, and indeed, even on the specific phase of the encounter where needed.