At Oscon, I highlighted the problem with many of today's cloud definitions and the attempts to pigeon hole cloud computing as a discrete technology, distribution or billing mechanism.
Such definitions unnecessarily narrow the richness of the field since Cloud computing represents a transition, a shift of many I.T. activities from a product to a service based economy caused by a quartet of concept, suitability, technology and a change in business attitude.
Whilst you can certainly describe some of the mechanics of cloud computing, NIST's latest definition does an excellent job of this, the mechanics do not provide the entire picture. It would be like describing the industrial revolution as :-
"The industrial revolution is a model for enabling convenient, consistent and high speed production of goods that can be rapidly adapted to meet consumer needs. The model promotes availability of goods and is composed of several essential characteristics (use of machinery, higher production volumes, standardised quality, centralised workforce) ..."
A working definition would fail to capture the fundamental transition at play, the history and the potential consequences of this change. It would fail to provide any understanding of what is happening.
An example of this lack of understanding and hence confusion would be the latest debate over what is or is not cloud. This has recently been re-ignited with Amazon's launch of VPC ("virtual private cloud").
In a service economy, a service can be provided by any party and the total of the services provided to an organisation may include both internal and external resources. For example, a company with its own cloud (provided with its own infrastructure, as in the case of an UEC environment) may combine these internal resources with external resources from a provider such as Amazon EC2. This is commonly called the hybrid model.
However, this is a service world and we shouldn't confuse provision with consumption. For example, the company in question is freely at liberty to consume such resources internally (as a private cloud) or to provide those combined resources to others (as a public cloud).
A quick glance at the electricity industry will show you that there exists a wide mix of internal and external resource providers and different models for consumption and provision. I can build my own power generator, top up with the national grid or even sell back to the national grid. The same myriad of different options exists within cloud computing.
Amazon's recent announcement is important because it further enables one of these options. For example, a company could choose to build a private cloud (for internal consumption) which consists of both internal (as in UEC) and external (as in Amazon's VPC) resources. Of course, there are many other ways that this same result can be achieved but none have Amazon's momentum in this field.
The debate over whether it is or is not cloud computing, is simply ... immaterial.