Can they? Yes, but success isn't guaranteed. Having 51% of the hashrate doesn't automatically mean they can succeed with an attack.
Will they? I seriously doubt it. A successful attack will ruin Bitcoin, and an unsuccessful one will ruin them. Neither scenario would put them in any good situation, unless these huge miners purposefully wanted to bring Bitcoin down.
The Ghash situation shows exactly the strength and vision of the Bitcoin community. Instead of using the majority hash to mess with the system the pool agreed to limit and individual members left, showing collective and individual responsibility and determination for making the potential of bitcoin a reality.
I'd say it's less about strength and vision, and more about how it will be advantageous for no one for Bitcoin to plummet. The system does work incredibly well though if viewed in that light.