Ghostcrawler has said on the official forums that Blizzard can see a world where Fury and Arms Warriors could tank. He goes onto say that Druids and Paladins would be more tricky as they have healing options too. Basically, he said that Blizzard would like to get away from trees for defined roles, ie the tanking tree or DPS tree, and be more like the Death Knight, whereby each tree could be used for whichever role they wanted to play.

Read Ghostcrawler's full post below, continued after the break:

  Blizzard Quote: (Source) 
 We like the way the death knight's talent trees are shaping up. Specifically, you can tank using good builds from any of the three trees. One may be slightly higher than another at any given moment in time, but overall the goal is that they are very similar and they are certainly more similar than the other three tanking classes.

We can totally imagine a world in which Arms, Fury and Protection all can tank using different styles. The Prot tree would just be more about sword and board style than tanking per se. Perhaps an Arms warrior would tank with a two-hander more like a DK does. I'm not saying we will 100% for sure go down that road, but it is interesting idea to think about. 
  

  Blizzard Quote: (Source) 
 It's harder with druids and paladins because some of the other trees can heal or are ranged. (You could probably come up with a Ret tanking system). Being able to tank and heal effectively with

To some extent druids are always going to be about being able to fill very different roles. While a DK tank might have a Frost tanking build and Blood tanking build that she switches between, I don't know that you would see many druid tanks switching between Bear build #1 and Bear build #2. Some will, and that's cool. But I suspect a lot of them will swap from bear to cat or from bear to Resto.

I could easily see a Prot warrior who wants two different Prot tanking builds -- a high threat build vs. a high survivability build for example. I could see a Prot warrior who wants a Prot tanking build vs. a Prot off-tanking build or even a Prot PvP build.

But I don't think this feature automatically locks us into Prot = tanking and Arms / Fury = dps if we don't want it to be. There is a risk that it allows us to be lazy: "Oh don't worry about making Frost mages PvE viable because they can always swap specs to Fire or Arcane for PvE." We would still like to get away from X being "the PvP tree" for every class. Likewise we'd like to get away from having tanking trees too, but it's harder or perhaps impossible for some classes.