In a nutshell, private clouds are Amazon-like cost-effective and scalable infrastructures but run by companies themselves within their firewalls.
Private clouds are about extending the enterprise to leverage infrastructure that makes use of cloud computing capabilities and is not (only) about internally locating the resources used to provide service. It's also not an all-or-nothing proposition.
It occurs to me that private clouds make a ton of sense as an enabler to enterprises who want to take advantage of cloud computing for any of the oft-cited reasons, but are loathe to (or unable to) surrender their infrastructure and applications without sufficient control.
Private clouds mean that an enterprise can decide how and how much of the infrastructure can/should be maintained as a non-cloud operational concern versus how much can benefit from the cloud.Private clouds make a ton of sense; they provide the economic benefits of outsourced scaleable infrastructure that does not require capital outlay, the needed control over that infrastructure combined with the ability to replicate existing topologies and platforms and ultimately the portability of applications and workflow.These capabilities may eliminate the re-write and/or re-engineering of applications like is often required when moving to typical IaaS (infrastructure as a Service) player such as Amazon.From a security perspective -- which is very much my focus -- private clouds provide me with a way of articulating and expressing the value of cloud computing while still enabling me to manage risk to an acceptable level as chartered by my mandate.