How much of amazon's revenue is generated by ec2?

Unless you are an insider it will be difficult to fins out. However, the following link gives you some information (http://www.crn.com/news/applications-os/226500204/amazon-cloud-revenue-could-exceed-500-million-in-2010-report.htm). They estimate that a $500 million revenue on cloud services (this could be compared to a total revenue of about $ 25 000 million a year.