Maybe not a killer app, but there are certain kinds of collaborative 'CRUD' apps that could benefit greatly from having versioning built into the database as a service.
For instance, how much of a functional wiki could one assemble from off-the-shelf parts? Edit, display, account management, templating, etc could all be handled with existing libraries in a wide array of programming languages.
The logic around the edit history is likely to contain the plurality if not the majority of the custom code.