No. If someone does manage to control 51% of the computing power of Bitcoin, he's much better off using that to mine bitcoins rather than "cheating". If he'll use it to cheat, he'll cause panic, make bitcoin drop their value and undermine the source of his own potential wealth. It'll be much better for him to play fair and just gain (a lot, with that much computing power) wealth by mining.
I guess that's right. Someone with enough computing power could harm bitcoin to the point it'll be unusable. But the people with those motives (government, banks) are much less likely to be part of something like that than people with financial motives.
