Doesn't the amazon cloud service pose a potential 51% attack threat?
Firstly, consider the costs:
Cost for Cluster GPU Instances runs $2.10 per hour, or $1.70 after maximum discount for massive purchase volumes.
Revenue from Bitcoins produced by Cluster Computer Instances runs from $0.02 per hour on down.
So economically, that's suicide.
But lets say, for whatever reason to prove this you could spend a half million dollars for the day and Amazon somehow happened to have enough excess capacity (which is unlikely they would).
So what does the ability to do a 51% attack give you?
Sure, you can refuse to include all transactions and grief bitcoin for a while. You probably aren't going to have enough excess hashing to be able to go back and rewrite the blockchain even a block much less two. So anyone with several confirmations is not at risk of any double spend. And because the network hashing level went up so quickly, anybody doing a transaction is going to be extra cautious ... waiting at least 6 confirmations, maybe more.
Eventually (after a few your hours) your budget runs out, and everyone continued to mine and now extends off of your last block, and life continues. Included in the next several blocks will be all those transactions you refused to include.
So ya, ... technically bitcoin is at risk of being griefed by a party with massive hashing capacity. But if it comes, it isn't going to be from Amazon's GPU clusters.