The first thing to keep in mind is that an Arms or Fury warrior can tank, even in TBC instances. It's been stated before that an Arms warrior is intended as the baseline of warrior tanking, while protection is seen as a more dedicated tanking spec and fury is the pure DPS tree. As it's worked out, arms talents are also highly effective in PvP, especially in arenas where you need to do as much damage as fast as you can before you're wiped out. The added bonus of the Mortal Strike debuff forcing enemy healers to burn more mana to heal and possibly running out faster helps contribute to the spec's popularity in PvP. But it's far from being a worthless talent tree for tanking. Talents like Second Wind and Improved Discipline are useful for mobs that stun or immobilize you, and there's no way to argue that being able to use Shield Wall more often is a bad thing. By speccing arms a warrior can be a credible tank while still maintaining the ability to apply decent burst DPS when needed.
A fury spec warrior might have more difficulty staying upright, but with the right combination of fury and protection talents, you can DPS very effectively AND hold aggro with abilities like Enrage and Flurry. A tanking warrior with a high crit rate who can use Rampage to increase his attack power liberally can get more aggro simply because he's doing more damage than otherwise, and Improved Cleave is never a bad thing when tanking multiple mobs (unless you have the bad fortune to be trying to tank them in the middle of CC, which can happen).
Obviously, both of these specs will require some talents in prot. But what they really are is an example of how warriors can do more than one thing in an instance. When the improvements to feral and paladin tanking were first implemented, both groups argued that warriors should welcome the change, as it meant they wouldn't have to tank every single time they did a run anymore. Which would have been a welcome change, had it happened more than it really has. The problem is, not many people want warriors to do more than tank. If a group already has a tank, many times there won't be serious consideration given to a warrior in a DPS role even by people who would have no difficulty arguing that warriors do more damage than they should. If warriors are such overpowered DPSers, why is it so hard for them to get groups outside of guild runs as DPSers? A DPSing warrior cannot simply flip a switch and become a main tank... his gear won't be optimized for it, and slapping on a shield won't make up for all the defense and stamina and other tools tank gear brings to a tanking warrior... but he can go from DPS to offtank to save a group in much the same way that a feral in cat form can switch to bear to pull a mob off of a healer. Taunt works no matter what your spec is. You can sunder with a 2h or two weapons equipped. No, it wouldn't suffice to actually tank the entire dungeon. But it can and often does save a group from a wipe.
Protection spec warriors will always be, and should always be, the kings of warrior tanking. But with ferals, prot pallies, and now DK's coming into the mix there needs to be more flexibility in terms of allowing warriors into groups, be they tanks or not. If the argument is going to be made that having other tanks allows warrior to not always tank all the time, then they actually need to be allowed to perform other roles, otherwise you're allowing them to not always tank by not letting them into groups at all. That way, it actually is good for everyone to have more options in tanking and DPS. For that matter, a group that contains a warrior and a feral can actually have options as to who will tank which boss or which groups of mobs. Is the feral a better choice for this particular pull, while you'll want to have him go catform while the warrior tanks the boss mob? Now you can. Would you rather have the prot pally tank the large pull while the mage AoE's them down due to his superior AoE and group taunt, but you need a little extra healing for the pull after that, which can be easily tanked by the warrior with his superior mitigation?
Too many folks are stuck in the mindset that a specific class always has to fill a specific role and stay with that role for the entire instance or raid. Classes that have more than one strength should be used to their fullest, and for a warrior, sometimes that means letting someone else tank. With the Death Knight incoming, we'll hopefully see more groups willing to let particular tanks work to their strengths, sometimes DPSing, sometimes tanking, instead of trying to fill a scorecard of roles.