Could the EC2 infrastructure be used to efficiently launch a 51% attack? This excellent answer from Serith states that one would need 31150 ATI 5870 graphics cards to launch a 51% attack. Amazon EC2 offers computers for rent for 2.10$ per hour. These instances have 2 x NVIDIA Tesla “Fermi” M2050 GPUs. Given that these are as fast as the ATI 5870 (I have absolutely no idea whether this is the case), and that Amazon has enough of these instances available, one should be able to launch a 51% attack for one hour for a mere 65415$. If the hash rate of the Bitcoin network varies over the time of the day or under such events as a power cut, then it could be cheaper.

Is this a potential threat to the Bitcoin network?
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.