A good list of considerations, but missing a crucial item: Type of clouds offered: Is it an Infrastructure-as-a-Service or Platform-as-a-Service?

Like the difference between managed services and a true utility, different vendors offer different types and apples-to-apples comparisons between types doesn't generally make sense.

For example, Amazon's EC2 service is an example of Infrastructure as a Service. You manage everything inside your Virtual Machine, separately license all the software, handle load balancing yourself, etc. In other words, you get both the flexibility and overhead of a bare VM.

Microsoft's Windows Azure is a much different kind of cloud service, and is currently the best example of a Platform-as-a-Service cloud. In Windows Azure, you manage your application instance vs. a virtual machine instance. There is no separate software licensing, all infrastructure management, load balancing, etc, is handled automatically by the cloud service. Similarly, with SQL Azure, you get a relational DB in the cloud as a service - no separate licensing for db software and it doesn't necessarily require a dba to set up and manage. In other words, your people can focus on the application and much more than the infrastructure management.

Actually I think that Forrester is underestimating the potential impact of 'cloud' specifically of payg services like Amazon's. The value's less in the immediate capex savings for test and dev systems, with corresponding asset utilisation increases, and more in enforcing standardisation of the infrastructure and improving the testing models to remove the increasing power of the 'operate' part of the ICT value chain.

The challenge in making the approach work is in controlling the end-to-end delivery of ICT, while outsourcing most of the expense, rather than losing control of the estate.

@anonymous: maybe for the sme world, but PaaS is less useful for grown up businesses as it creates too much proprietary lock in, is hard to get to from the existing estate (either migration or integration) and it has a high risk of platform incompleteness.