With recent advancements and announcements in the Industry, its clear that there are two emerging models for taking an Enterprise data center into the Clouds.
The first approach "the private cloud", requires an enterprise to purchase "Cloud in a Box" software such as vCloud and vSphere along with virtualization software provided by vendors like VMware. VMware is going a few steps further up in the stack with acquisition of SpringSource which will enable their existing and future customer base to seamlessly develop, deploy and manage applications in VMware based Clouds. The Private Cloud is applicable when the cloud is confined to an enterprise owned data center and provides a great way to scale the existing virtualized customer data centers by adding the flexibility and utilization efficiencies of a Cloud. Vendors like Rackspace and GoGrid are building Managed Private Clouds for their enterprise customers using this approach.
While a private cloud offers the CIO the benefits of a Cloud architecture, unleashing resource management, utilization, and on demand scaling capabilities, it still does not meet the goals of a pure Cloud as it only offers limited elasticity and does not eliminate capex. The enterprise still needs to own and manage all the resources. Werner Vogel has explained this very eloquently in his blog here. Nevertheless, it enables CIOs to better manage existing resources by means of metering, billing usage, and charge back to other business units.
The second emerging approach is that of Hybrid architectures where enterprises can extend their existing IT infrastructure to leverage on-demand resources of an external cloud thus adding scalability on demand. The enterprise continues to utilize their existing data center and augment it by offloading certain types of usage to an external cloud. They can also use this approach for handling the occasional burst loads, and not having to over provision their own infrastructure to meet peak demand. This approach is in line with Amazon's announcement of their Amazon Virtual Private Cloud (Amazon VPC). While it has its limitations, its a solid first step in this direction.
The two approaches are complimentary, and can be combined towards building Hybrid Clouds where resources are moved between multiple Clouds seamlessly. However, I see a need for standardization of protocols and APIs offered by the clouds from different vendors before we can offer this level of flexibility to all users.
[Fig. from Wikipedia: Cloud Computing]