My guild's never dropped (or even attempted for that matter) Sarth+3, but I was the MT on Sarth for our first downing of 2 drake Sarth. I didn't go any specific spec, just my standard Blood tanking spec.
My question to you all is with Dual Speccing coming out, I'm not sure which of these 3 tanking specs I've created will complement each other the best. Keep in mind that my guild has a plethora of DKs and usually takes 4 to 6 per 25 man and I'm the only Blood there so IIT will be on the target at all times as well as Ebon Plaguebringer:
Blood
This will for sure be my main, single target threat spec. I picked up Spell Deflection as I see it proc on more spells than I think people realize so it's simply a preference of mine. Also I chose Glyph of Death Strike and Dark Death as I plan to replace Obliterate in my rotation; and Dark Death coupled with Sudden Doom and Morbidity should be some healthy single target threat. I did not pick up SoB as I find as a blood tank, I rarely use my RP on anything but rune strikes and with there always being a blood or death rune available, don't have the extra time in my rotation to spam DC every few seconds.
Frost
My goal with this spec is to try and become more magic resistant yet still maintain a lot of AoE tanking capability. With (from what I understand, not actually experienced) Frost's better single target threat generation, this should hopefully still allow me to tank bosses. More specifically though, I really wanted to get my magic mitigation high.
Unholy
This spec I plan to be my primary AoE tanking spec, but I remember it was quite useful for magic damage mitigation as well. I don't have much to say about this as I find it to be not much more useful otherwise, yet find myself unable to simply disregard it. I did not pick up Desecration as I expect Ulduar will require more movement than we've seen thus far.
Any help or suggestions would be appreciated.