Since I'm always referring people to that site for some good how-to class threads, I thought I'd post the direct links here, so save everyone some digging around:

Quote from elitistjerks:

"Some people will try to tell you that pre-shielding a tank on a pull results in less threat, or a rage-starved tank. Unless you're running something with a fully kitted out mitigation-gear T6 tank (basically passive-crush-immune gear), this is patently false. Every mob of significance in end-game raiding hits for 3k+, and a PW:S+PoM will basically eliminate the first attack from the table, without causing *any* threat to you, as a healer. It is far better to keep the tank alive through the first 2-3 attacks than to worry about how much rage he is getting from those attacks. Realistically, almost every boss is pulled with Misdirect + Earth Shield + PoM + trinketed Shield Slam (warriors) anyway, so rage isn't really an issue when the boss is locked onto the tank and will be beating the crap out of him Very Soon Now. If your tanks really suck and need that rage to build threat ... then you're probably going to pull it off them on the first heal anyway, which negates the argument."

Found that pretty interesting.

Twistd

Forgot, shoulda mentioned this was for boss fights, not trash.

With 700 mana regen out of combat, most of the time the mana it took for the shield (around 400 mana) will be regained by the caster before the tank gets to the boss. And with global cooldown of 1.5 secs on a bossfight where  they are hitting for 3.5k or more per hit, a shield would be the worst thing to do, as it only mitigates a small amount of the hit and it would be better to cast a 1.4 second (with haste) flash heal, that heals for 2500, rather than wait on a global cooldown (1.5 secs) of a shield that only mitigated around 1000 damage. Tank is still taking more damage than a single healer can give, but its about the time to have other healers help.  As for the 15 second "weakened soul" debuff, you'll know in 15 seconds whether or not your tank will survive so it doesn't make much difference.

I doubt that i would shield the tank either, mostly because i dont want to have to go into a logistics fight and explain why, how it works, etc over and over again, when we're not really at a point where it would be useful, (except perhaps za), nolorakk was hittin for like 9k a hit at some points.