We do have a fix for this ready, however, it will require a client-side patch to address. We're in the process of building a small patch to fix this and a few other issues which also need client updates. You're likely to see what we call a fast patch in the near future (a la patch 4.1.0A in recent past: http://us.battle.net/wow/en/game/patch-notes/4-1-0a).
Sorry, it's been a while since we've communicated anything about this. Our team has been very busy lately working on a lot of cool things, as well as finalizing plans for BlizzCon, but we're trying to keep our eyes on the forums as much as possible in the process. :)
As knee-jerk as many of the reactions are, I have to say that the wait on this when a fix is apparently available is getting tiresome. One of the things about Cataclysm was supposedly "more, faster patches". Release this as its own patch this coming Tuesday, release the rest next week or when it's ready.
Waiting for a bunch of trivial and very area-specific fixes to be prepared to fix something that's borderline gamebreaking is very frustrating. Firelands is very add-focused, having Tab not work and abilities deciding they'd rather switch targets back to dead mobs instead of what you're trying to target is awful.
I have to imagine as someone who is a new tank trying to level and gear through normals/heroics for the first time at 85 this has to be intensely aggravating at times. I know from personal experience with a newly-85 warrior that the situation can get completely bizarre. Heroic throw one target, tab to a different to charge, hit charge, and charge runs to a third different target for no apparent reason.
This is in addition to the simple extremely random nature of tab-targeting in general. As a function that was largely popularized in MMOs through WoW and one that is so integral to gameplay, I would think this would be a higher priority.
EDIT: By the way, seriously, the people going "omg lost subscribers WoW is dead" are ridiculous. The game still holds 11 million subscribers. It's been the most successful and popular P2P MMO for nearly 7 years. I'd be more concerned with the fact that the industry hasn't come up with any reasonable competition in all this time. To expect a MMO to not 'slow down' after nearly 7 years is unrealistic. Before WoW you'd be hard-pressed to find players that held a MMO subscription for more than a year or two, nevermind a huge playerbase that's been playing for 6.