Perhaps some of the disconnect is that it's not easily explainable to non techie types without going in to more detail. The Gmail/Office example you gave holds true for any web app, regardless of it is hosted on Force.com, a $9/mo hosting plan, or a server I run out of my apartment. Computational power as a commodity resource is a little more in depth than most people probably care to go, but it leaves many with the impression that the internet is the cloud.