I have a desktop application which uses a local database. (This can be SQLite, SqlCe, PostgreSQL or any other database I will be able to install locally, I haven't decided which one to use yet.)

The plan is to achieve the following:

A user can subscribe to some kind of cloud service. If he does his local database should be synced with the online database (one for all users, or one per user, whatever the easiest solution is) so he will be able to sync his local database data between multiple PC's, can access his data online. (Much like dropbox does for files.)

What is the best, easiest (and preferably cheapest) solution to achieve this? I am looking into DataObjects.net but I can't find much documentation about their Sync feature. Or, are there other alternatives? For example, I start with some kind of cloud service which allows local caching and use the local caching for users who do not subscribe to the service.

Any pointers, tips or experiences are welcome.