Leveraging the cloud means thinking beyond standard models, business processes and technology. The cloud is inherently service-oriented as its users are reaching out to consume and expose web services. Being able to link together services sounds much like service-oriented architecture (SOA).
Think of the Enterprise Service Bus (ESB) you've deployed at AWS as an Application Programming Interface (API) conduit, a control point to provide the flexibility to allow business units or teams build composite services. This matches up to a large extent with my working definition of SOA: loosely-coupled services with well-defined interfaces that provide functionality and can be shared or reused across and outside the organization. These services can be discovered through a registry/repository or other directory, and can be assembled and dis-assembled to meet current business process demands.
Over at InfoWorld, the author notes that the architecture of Salesforce.com's Force.com PaaS (platform as a service) embeds a lot of SOA approaches. For PaaS platforms to be useful, they must be service oriented to some degree.