http://techblog.netflix.com/2010/12/5-lessons-weve-learned-using-aws.html
Notes from netflix tech blog about some key points of their EC2-based infrastructure design. Note that netflix is not affected by the current EBS problems in AWS US-EAST region. Their key point seems to be to equally split your infrastructure between 3 AZs (availability zones) and run at ~30% capacity, so if 2 AZs fail (as we have now) you would still running in one AZ @ 90%