I've been thinking about this lately - about giving all of your data to a third party and hoping they don't go out of business or erase your account (for whatever reason).

Issues of software freedom aside, I think the key to making people trust the cloud is to make the data contained in the services more transitive. Essentially, creating a standard Import/Export for the web. i.e. - If I decide to switch to vimeo for my web video needs, I'd like to be able to import all of my videos from youtube without much fuss (or make a local backup). That's not a great example, but it's the general idea.

Hmmm...after I think about it, this pretty much boils down to: "let me get at my data in a format that I can read."