If you are able to afford them, I would recommend EngineYard's Cloud, which is built with EC2 AMIs. They are around 30% more expensive than EC2 on-demand rates last time I checked.
ec2onrails is a good start, but the project has stagnated, and I don't believe there is a release version that uses EBS for the database files, although the development branches do have that feature. I have used it and it worked well. You will definitely want your database on EBS for snapshot and mobility reasons.
There are a few others floating around but I have not looked at them.
I ended up rolling my own using Passenger, which was not very difficult and left me far more comfortable. Your requirements are pretty basic.