We support S3, Dropbox, CX, Box.com, DreamObjects and local file system. Two of which were contributed by the community. The point being, this list isn't controlled by any single person or entitiy.
> However, the difference lies in the promise from app.net that they won't arbitrarily shut off your app because they feel like it or because it somehow competes with their own service.
Perhaps I'm in the minority that doesn't trust promises of a company with valuable data (photos, for me). If it's not valuable data then why care at all (my tweets).
> I'm going to incur a switching cost
For hosted accounts we provide a migration option. All your photos are seamlessly moved from Dropbox to S3 and all your links and mobile apps work as they always have. http://i.imgur.com/wWUc5uA.png
That being said, someone should write an App.net file system adapter for OpenPhoto :).