Hey there. I've been Off tanking in the newest Cata dungeons, and occasionally playing the role of Main Tank. I was geared up with Stam and Agility, and recently switched to adding in more Mastery. Bear tank Mastery is a damage reducer, so this helps the healers. I communicated with the healers from our group and specifically the healer who was assigned to me. I wanted to get first hand knowledge of whether or not the switch helped me. According to the healer, it was much easier to keep me alive. My HPs are about 176K raid buffed which is plenty.
I suggest going to a site like
www.askmrrobot.com to check your gear and optimize where you can. Now, if only I can get those jewel crafters to stop charging 9K for a Queen's Garnet. ARGH!