Dual Specs has been a very hot topic in the tanking community. There has been a lot of talking about taking two tank specs vs a tank spec and a dps spec (or healing if you are a Druid or Paladin). I think a lot of this discussion has mostly come from the warrior tanking community and Veneretio at TankingTips.
Personally, I do not think that there is a right or wrong answer. I think that the decision should be based on what you are going to be doing in most fights. If you are going to be the main tank for every single fight, then by all means, take two tank specs. Now if you have decided on two tank specs, what do you choose? Do you keep the thread oriented Deep Wounds Spec, which is a very nice threat spec and very good for trash as well? Keep in mide with Deep Wounds that you can cause problems if CC comes back into play in Ulduar. If you are looking for an adventure, a very situational tanking spec would be the Unrelenting Assault Spec. If a tank is taking two tanking specs, then he will be certain to have a survivability spec.
On the other hand, if you are mostly off tanking, then you make do as I do and keep Deep Wounds for good trash/threat and pick up a nice fury build for the times you will be needed to DPS. Spinks has a very nice post on Fury for Begginers for those of you who have been tanking all expansion long and would like to get in on some DPS action. I will be going with a standard Fury Raid build for my second spec to help out with DPS when my tanking is not needed. This would be the recommended set-up for most tanks.
All of this depends on the situation you are in. If you are the best geared tank and expected to tank all of the bosses, then dual tank specs is not a bad idea. I dont think that anymore than one tank per guild should have two tank specs. If the fights are going to be much tougher in Ulduar and DPS is going to be a big deal, then fights that only need one tank are going to be gimped by a bunch of tank DPS.
Check with your guild to see what is needed. If they dont have an opinion, then go with what you want. Tanks rejoice, you now dont have to suck at DPS......unless you still want to of course :)