Head in the Cloud, Feet on the Ground
Eugenio Pace and Gianpaolo Carraro from the architect team have just published and article in the latest Architect Journal ( #17) which is entitled "Head in the Cloud, Feet on the Ground" which talks about the possibilities of the moving IT assets from the on-premise data centers into the cloud, and the compromise vs cost discussions which need to happen over a multi-year period.
Here is one of the diagrams from the document that I like, which describes the process the a pharmaceutical company could take to transition some of their assets over time to the cloud.
The overall tone is that there is no one golden solution, but there would be a hybrid of on-premise and in-the-cloud services evolving over time.
Clearly some services move to the cloud a whole lot sooner. These include messaging, where there is proven scale and control, and transitioning your Microsoft Exchange system to the cloud would give you considerable cost savings with no loss of features ( in fact you might get more ).