Synchronize SQLite database and Dropbox datastore
FYI, the Datastore API 3.0 beta includes a preview feature called "local datastores" that will hopefully obviate the need for this. See the blog post here announcing the preview feature: https://www.dropbox.com/developers/blog/97/sync-api-and-datastore-api-30-beta-release.
Finally I have this implemented. Sources can be found here. Key class is implementation of Dropbox storage. App uses storage interface and works fine with both implementations (database and Dropbox datastore). Both implementations cache results of frequent queries. Only one thing I concerned about is performance. I haven't yet tested and compared performance of data queries and storage content modification calls for these implementations. But I would say that app works smoothly and I haven't noticed any lags.