EC2 would be prohibitively expensive to attack bitcoin but would be well suited against so called "GPU unfriendly" block chains. Against bitcoin EC most likely usage would be to "top off" an attackers hashing power. If a hypothetical attacker built a massive hashing farm but was slightly short of 51% hashing power the attacker could use EC2 instances to push the attack "over top".
EC2 is ill suited as primary source of an attacker's hashing power because EC2 GPU prices are expensive, the GPU Amazon chose is poorly suited for hashing, and the number of instances is lmited (Amazon only has so many GPU instances). CPU instances are significantly cheaper but they have on average 1/50th the hashing power of average GPU which significantly increases the number of instances necessary to achieve 51% hashing power.
For so called "GPU-unfriendly" block chains (litecoin, tenebrix, solidcoin, etc) use of EC2 instances is far more efficient and economical. Since these chains chose to exclude GPUs (a dubious decision) they have in effect removed the performance advantage between the hardware used by honest nodes and the most cost effective hardware available to attackers. Each honest node is now significantly less powerful (being limited to on average a single CPU) and CPU power (in a variety of forms) is much cheaper and easily accessed by an attacker. In essence these chains have commoditized the hardware used for hashing and as such left themselves vulnerable because large amounts of CPU hardware can be obtained for very cheap if only needed for a short period of time (like in a 51% attack).
Although the problem exists in all "GPU unfriendly" chains lets take a closer look at one chian, Litecoin. Litecoin's hashing power is roughly 30MH/s. To defeat that would require only 6000 Amazon Compute Units (virtual CPU roughly equivelent to a modern 1GHz Xeon CPU). The largest Amazon instances have 30 compute units. Thus it would only require 200 of the largest instances or roughly $300 per hour at current prices to achieve 51% of Litecoin's network hashing power.
Every other "GPU unfriendly" coin faces the same problem. By excluding high performance GPU they have made it easier for an attacker to leverage large sources of easily obtained low cost CPU power putting the network at risk