There are several cloud service providers. But, if you're going to design an app that runs on their infrastructure, you have to have confidence that they are going to be around for a while, and that they are going to continue to offer the service. For example, Google might decide that AppEngine isn't profitable and close it in a year (like many of their non-profitable "20%" projects). Cash strapped startups might not make it through the current economic downturn and be forced to close down.
So, who do you trust in the cloud?