Actually, theoretically it is.
If someone is to control 51% or more of the network power then they can do it.
And my guess is that when (not if) the time comes where only a handful of corporations (and/or entities) will be left (and able) to mine efficiently we will see things like that.
It's only a matter of time IMO.

Right now, the 51% attack is prevented by the fact that the average Joe is still (kind of) also able to mine and they do it P2Pool.
What happens when it's not profitable (at all) for the average Joe to mine?