We've known for a while that the developers have wanted to remove crushing blows, but now it looks like they're starting the process for good: Ghostcrawler has confirmed that crushing blows will now occur only when a mob is 4 levels above you, rather than 3. Since "Boss" mobs are automatically 3 levels above the max level, that means that if you're running level appropriate content, you should no longer have to worry about crushing blows at all.
As Ghostcrawler explains it, crushing blows were originally meant to discourage you from attacking high level mobs and to make bosses more challenging. Now, they serve only the former purpose, and Blizzard is working on other ways to make bosses challenging without making them so random.
This change definitely comes as welcome news to Feral Druids, who rely on high armor for tanking and are unable to reach uncrushability under current talent and gear choices. Paladins and Warriors, who use gear and Holy Shield or Shield Block to reach uncrushability, are probably less enthused. Death Knights are still working on finding their tanking groove, but since they can't use shields, it's likely the removal of crushing blows will only help them.
Those who are a bit concerned about the change may want to take heart (or find dread) in Ghostcrawler's words in another thread: Raiding is changing. In Wrath of the Lich King, they are hoping to have all 4 tank classes be able to step and tank straight up into the 25-man level (which means, in theory, that Paladins and Warriors should be getting stuff to compensate for losing uncrushability, and Death Knights should eventually have the gear and innate mitigation and avoidance to tank).
They're looking to reduce the randomness of raid encounters as well. This is their philosophy behind the removal of crushing blows as a boss mechanic. In addition, they want to make tanks do more DPS, make threat easier to manage, and make buffs to affect raids more but stack less. All of these additions, of course, should greatly change the face of raiding in Wrath.
Of course, all of this change is a bit much to process, and I'm sure that regardless of what Ghostcrawler says, a lot of tank classes are still a bit worried about their place in both groups and raids in the upcoming expansion. That's understandable, but luckily, this is a Beta. Hopefully, Blizzard keeps these new philosophies in mind as they continue on, and by the time Wrath goes live, we have 4 solid and viable tank classes, and a more streamlined, less random raid experience that still manages to be fun, challenging, and rewarding.