I've been tanking trash mobs and heroics unholy since I started playing my DK , but have recently been swapping to blood for raid bosses in ICC only. For my playstyle, the threat and rotation from unholy is far superior to frost. I can literally run into a pack of 40 mobs, drop death and decay, spread my diseases, and blood boil my way to enough threat on all of them. I found howling blast lackluster and the frost rotation clunky. That is (was) the cool thing to me about DK tanking, finding a style that fits you or the particular fight .. it's a shame we are losing that come Cataclysm.

Like Daniel said, you do have to gem and trinket for avoidance over health if you plan on raiding with it, but if you can manage to keep bone shield up, you will indeed take less incoming damage than any other tank, in any other spec. I still think though, for boss fights blood has an edge with the higher health pool, increased healing, and self healing. But unholy tanking will always be my one true love for everything else.

If they're moving Bone Shield to Blood in cata I sure as hell hope they're gonna move Howling Blast as well.

Say what you will, glyphed Howling Blast is one of the best AoE Threat spells in the game, comparable to a paladin's consecration back in BC.

Howling blast will most likely stay in frost as its 51-point talent. Blood will likely have talents revolving around DnD and Blood boil.