As reported here last week Amazon's Elastic Computing Cloud (EC2) service is now out of beta and available for general use; now, a new service is out which begins the mind-bending journey of layering on-demand service on top of on-demand service.

Coupa intends to deploy an on-demand electronic procurement system on Monday, which relies not only on Amazon for computing power by way of EC2, but also for backend storage using the company's Simple Storage Service (S3).

Coupa has been using the EC2 beta for some time in proof-of-concept deployments and apparently has been pleased enough with it to count on it as a full-time backend for their mid-market aimed product.

There have been a number of other commercial services using Amazon successfully to provision on-demand services during the beta, but this seems to be an indication that the trend is accelerating and we're likely to see more and more business models based on leveraging virtual infrastructure that is not even operated by the company offering the service-utility computing finally coming to maturity.