Recently I saw a presentation from a very skilled Microsoft guy about Cloud computing. He was really good at presenting the message, telling about the cloud, about Microsoft products etc., but his terminology definitions definitely differed from mine.
Immediately, I looked up cloud computing on wikipedia, which corresponded very well with my own perception of the concept. Several years ago, I was in charge of a hosting center with a cloud computing system. It was basically just a hosting system, where multiple customers shared a PC, but it was able to move customers from one computer to another, when more power was necessary. It was nowhere as smart as the systems we have today, but to the customer, it was a dynamically scalable resource provided as a service over the internet. Some may say that cloud computing requires the ability to scale above what one server can deliver, but seriously, that's just a service specification, and for many users it is irrelevant.
If you look into existing corporate data centers, you can also see other kinds of "Cloud systems" - for instance, big VMware servers running a large number of virtual servers. To the departments, that buy these servers, and access them via TCP/IP, it is a style of computing that is dynamically scalable, virtualized and provided as a service over the internet protocol. The users/customers don't need to have knowledge of, expertise in, or control over the technology infrastructure in this system, that supports them. The VMware systems can move a virtual server from one physical server to another, moving the IP address etc. without interrupting online services and without closing TCP connections.
Does the term cloud computing describe the VMware system? Maybe. Or maybe not. But Google Engine or Windows Azure is not a revolution - to some people it's not even new stuff. 10 years ago, Microsoft may have hijacked the term and changed it to its own favor. Today, they can bend it, but not as much as before. Online, updated definitions of terminology, like wikipedia, make this a lot more difficult. I am sure that the definition of cloud computing will change in the near future, to specifically include systems in your own data center, but the new thing is, that the definition will be changed by nerds, using a common reference in wikipedia. It will not be changed by a single company's marketing department.