As I mentioned previously, I've been playing with a new tanking spec. Here is the latest version of my frost tanking spec.
I went for more damage talents than you normally would grab for frost. Why? (Tubes!!) My guild has been having some difficulty with Ignis, and by swapping around a couple points I can now offtank and shatter brittle constructs in the same spec.
After playing more with forst, I really like the high single target DPS, and Unbreakable Armor is excellent for tanking the adds on Sarth+2.
Actually heading into Ulduar to put it to the test in a little bit, an edit to this post to come once it has passed the trial by fire, pun not intended.
Edit: Okay, I'm back from wiping on Ignis about 10 times. My new spec is much better for that fight than my old one. I'm starting to see that Frost is generally better for boss tanking, while my Unholy / Blood build works more effectively on trash or crowds of adds.
Being able to shatter the Iron Constructs and take the hit from them exploding did wonders for our ability to consistently get them down, particularly since the DPS that should have been shattering spent most of their time dead and/or in the slag pot.
We went from wiping with Ignis over 80% to wiping with him under 20%. A marked improvement if you ask me.