It is true, we're initially targeting providers that will give you a "server via API call". When you're developing against AppEngine (and I imagine Azure), you're committing to that platform, because it is very very specific. Whereas when you develop against Amazon or Slicehost, the server is the common unit ... which you are free to do whatever you like with.

That said, I think there is a lot of room for a similar project with storage. Amazon S3, Rackspace Cloud Files ... and maybe Azure will fit in there?

As for libcloud, we're trying to provide a DBI like thing for cloud server providers. It's not the end-all solution, but it is a clean first step towards cloud portability.