Rather than play link ping-pong, go read James Urquhart's post on the topic titled "The argument FOR private clouds
" which features the various positions on the matter.
What's really confusing about many of these debates is how many of them distract from the core definition and proposition served by the concept of private clouds.
You will note that many of those involved in the debates subtley change the argument from discussing "private clouds" as a service model to instead focus on the location of the infrastructure used to provide service by using wording such as "internal clouds" or "in-house clouds." I believe these are mutually exclusive topics.
With the re-perimeterization of our enterprises, the notion of "internal" versus "external" is moot. Why try and reintroduce the failed (imaginary) Maginot line back into the argument again?
These arguments are oxymoronic given the nature of cloud services; by definition cloud computing implies infrastructure you don't necessarily own, so to exclude that by suggesting private clouds are "in-house" defies logic. Now, I suppose one might semantically suggest that a cloud service provider could co-locate infrastructure in an enterprise's existing datacenter to offer an "in-house private cloud," but that doesn't really make sense, does it?
Private clouds are about extending the enterprise to leverage infrastructure that makes use of cloud computing capabilities and is not about internally locating the resources used to provide service. It's also not an all-or-nothing proposition.
Remember also that cloud computing does NOT imply virtualization, so suggesting that using the latter gets you the former that you can brand as a "cloud" is a false dichotomy. Enterprise modernization through virtualization is not cloud computing. It can certainly be part of the process, but let's not mix metaphors further.
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.
Further, there are some compelling reasons that a methodical and measured approach migrating/evolving to cloud computing makes a lot of sense, not the least of which James has already mentioned: existing sunk costs in owned data center infrastructure. It's unlikely that a large enterprise will simply be able to write off millions of dollars of non-depreciated assets they've already purchased.
Then there are the common sense issues like maturity of technology and service providers, regulatory issues, control, resiliency, etc.
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.
A model that makes sense to me is that of GoGrid's "CloudCenter" concept which I'll review under separate cover; there's definitely some creative marketing going on when discussing the blending of traditional co-location capabilities and the dynamic scalability and on-demand usage/billing of the cloud, but we'll weed through this soon enough.