antirez's "How fast is Redis?" page (http://redis.io/topics/benchmarks) draws some attention to the fact that Redis gets much slower when virtualized. He mentions that a state-of-the-art VMware hypervisor will cut Redis's performance by a factor of two.
EC2's performance is considerably worse than a state-of-the-art hypervisor.
For comparison, I just ran the same benchmark the article describes on the desktop PC I'm currently using to post this. It cost me $800, last year, and that includes the video card. It gets an average of 881,670 OPS, which is over ten times faster than an m2-2xlarge machine in this benchmark.