There are plenty of definitions of the "cloud". Most of them include aspects like services which are provided via the internet and which are highly scalable. But the discussion about terms like a "private cloud" proves that this is a somewhat insufficient definition. Depending on the definition of a "private cloud", these services might be delivered via a private network.
The insufficiency becomes obvious as well with respect to some of the aspects of the cloud. There are so many different types of cloud services that there are for sure some which, for example, are so specific that they don't need to be highly scalable - for example cloud applications which are devoted to a specific target audience with only few members like for example airlines or rail operators. There the scalability is automatically limited and not somewhat infinite, like often is assumed as a requirement for cloud services. And there will be many services devoted to much smaller groups (with respect to the size and number of members).
From my perspective, the essence of cloud computing are the services. Services are defined on various levels, from pure computing power up to very specific applications. These services are provided by someone. They have to be well-defined so that they can be provided by different providers and the switch to another provider is supported. This definition goes well beyond today's definitions in IT Service Management. It has, for example, to be defined, where (geographically) a service can be hosted - due to legal reasons.
Given that a well-defined service which can be run virtually anywhere is the core of cloud computing, it becomse obvious that terms like "private cloud" are just marketing fuzz. In fact there will be only one cloud with different operators, from internal data centers to external cloud providers. And by the way: Where should be the borderline between "private" and "public"? The (diminishing) perimeter of an organization? The fact that a partition of a data center in the cloud is used? A physical machine or a virtual machine? Actually it isn't possible to define that in a valid way.
The real value of cloud computing is that services can be consumed from different providers and that providers can be changed - sometimes pretty easy, sometimes with a little more efforts. That might be an internal or external provider, but you shouldn't care about in case that the requirements are fulfilled (which could as well mean that it is mandatory to provide a service internally).
There are many open points around cloud services and the related standards today. In case that we have defined that a specific service consumed in the EU has to be hosted in the EU - how do we avoid that the data is sent from Paris to Berlin via New York which might happen in practice? Obviously, a lot of work has to be done around standards, around service descriptions, around management tools at any level. But despite the shortcomings we observe today, the cloud will become reality and IT will be run and managed differently from today. There are far too many advantages in cloud computing.
We will discuss many of the topics around Cloud Computing, the opportunities, business drivers, standards, service management and so on at Cloud 09 in Munich in November 2009. Take part in these discussions!