But what exactly is that promise? That your data will be safe? Really, in the hands of a stranger? Same for confidentiality: a secret known by two people isn't a secret. No, I think the real problem is service availability: a Web app makes an implicit commitment to work for you wherever you are, and whenever you need it. That's why Twitter users were so upset last summer; they didn't lose any data, but neither were they able to do what they wanted - heck, maybe needed - to do, while the site was down.
And that, I think, is the real danger of cloud computing: Web apps are single points of failure. If I lose my Internet connection, or if my computer crashes, that doesn't hurt anybody else. But if some online service fails, a lot of people are affected, all at once. It's like a flood, or a fire. Oh wait, no, it isn't. The right analogy is a volcanic erruption. Which raises a serious question.
Is it really wise for us to move en masse to the sides of active volcanoes?