I recently hit the threshold of achieving an iLevel of 378 worn gear in my tank-set which makes me pretty happy as it more or less insulates me from Firelands drops. There are a couple of items I would love to swap in and out of course - the Mastery trinket I'm using isn't perfect, and my helm isn't ideal, but with a 4-set bonus and full block-cap with raid buffs, I think I'm pretty pleased with where my stats put me.
Here is my in-game character sheet with self-buffs, no flask or elixirs, but with mastery food:
This essentially gives me a mitigation rating of 101.23% - which is very close to 102.4% to achieve full block-cap. If I wanted to get to block-cap purely on my own buffs, I could probably use a Mastery Elixir and swap out some of the gems I'm using. Keep in mind, this is without things like Horn of Winter and the Dodge proc from the Windwalk weapon-enchant, which push the Parry and Dodge numbers higher, so in-combat, I'm typically at or over the block-cap.
I'm enchanted as per normal, nothing special there, and I'm gemming to get my bonuses using Mastery + Stamina gems in blue slots, instead of full Mastery gems, to pick up a bit of Stamina as I was getting a little low compared to my Death Knight co-tank. What is interesting is that I'm using a Mastery trinket and a Dodge trinket with some of the Dodge forged into Mastery, just to balance mitigation stats a bit. My Epic Jewelcrafter gems are Mastery and my ring Enchants are Stamina instead of Strength (for Parry). I'm not using pure-Stamina gems yet, and I'm not using a Stamina trinket either.
Now keep in mind - this is purely in normal Firelands gear. Once I start picking up some Heroic gear, that block-cap will be over the magic number, and I will start swapping Mastery for Dodge and Parry further reducing the incoming damage.
I've long since given up my dreams of being hit-capped as a tank even though I still hate missing, so the Dodge/Parry sum will start to climb and the Block chance will draw down. I don't know where the pure-mitigation cap will be - maybe 35%? More than 40% with 4.3 heroic gear?
That's just silly at some point, where nearly half the incoming swings will be missed, dodged or parried. Tank damage at this point will drop and that's one of the reasons we've seen things like Chill of the Throne and Sunwell Radiance in the past to reduce tank mitigation in the last tier of raids.
And that was without the benefit of Mastery, which has gotten, frankly, just too good for Paladins. Are we going to see anything like that in the Demon Soul raid? Who can say.
I'm sure part of this is the fact that I've just gotten lucky with drops, and I'm sure the folks who've been farming heroics for a while now have a lot better gear and their stats look a lot better - but point is, even in just normal Firelands gear which is easily accessible at this point to any semi-serious raid group, paladin tank stats are kind of freaking crazy right now.