Could the EC2 infrastructure be used to efficiently launch a 51% attack?
An NVIDIA Tesla “Fermi” M2050 is not as fast as an ATI 5870. According to the wiki, the NVIDIA card makes around 70Mh/s and the 5870 makes around 400Mh/s.

If they were similar and if Amazon had that amount of cards available for rent, then it would be possible. Just don't forget that you would be paying $65415 per hour to sustain an attack that will not give you that much power over the network. See this question for an explanation of what the attacker could do.