This has been the case in World of Warcraft since its debut. Heck, thanks to Indalamar, warriors got nerfed before the game went live. Balance is ephemeral. Your class may be on top one day, but your day will end. Anyone who's tanked for the past six years can attest to the roller coaster of which class is best at which aspect of the role. There was a time where paladins were the undisputed kings of AoE tanking, a time before Death and Decay or Blood and Thunder.
This doesn't mean some classes aren't unfairly stronger in specific situations; it simply means that perspective must be maintained. Blood DKs have risen to dominate tanking in this expansion, especially in Firelands and Dragon Soul, because of the mechanical advantage they gain from Blood Shield. Since Blood Shield absorbs damage, the bigger that Blood Shield gets, the more predictable the damage and the easier the job of the healer. (Edited to correct an error) Is it unbalanced? Yes, it is. Does it matter? At this stage of the game, no. The time to fix it for Cataclysm has passed. At present, any guilds that could connive to switch tanks have done so; the rest shall remain as they are.
There are absolutely imbalances in the game's design, artifacts of the rather astonishing game of Jenga the designers have to play with every single ability and talent and all the itemization. Itemization in particular always stresses the game's balance to its limits. Ten DKs couldn't run Firelands when Firelands was current content. Yes, they can do so now that they significantly outgear it, but does that matter? It's neat, yes, and we're all envious of it. But it's not significant. Back when paladins and druids were soloing content the rest of us couldn't dream of, that was technically a balance issue, but it again wasn't a significant one.
We have to consider balance in terms of active, at-level content first. My warrior goes fury to solo old content, and thanks to how Victory Rush works, I can solo Wrath content in some cases more easily than I can BC content. This is an oddity, yes, but again it's not a significant one.
With all of that coming, it's far less likely that there will be a patch redressing any changes made in patch 4.3, whether or not they're unbalanced. Work is dedicated to the goal of balancing the entirely new scheme that's coming, not trying to redress imbalances in a system that's about to be tossed out.
What is possible and even probable is that once we've passed a tier or two of raiding and gear has a chance to inflate, the systems cracks will begin to show, as they always do. What you least expect is what is bound to happen eventually. Balance is a goal, something to attempt. It will never be achieved until the game stands still.
World of Warcraft: Cataclysm has destroyed Azeroth as we know it; nothing is the same! In WoW Insider's Guide to Cataclysm, you can find out everything you need to know about WoW's third expansion, from leveling up a new goblin or worgen to breaking news and strategies on endgame play.