FirebirdSQL is pretty close to what you're talking about... it's library can do either embedded or you can run a shared server instance. It's really pretty neat, but on the one side, PostgreSQL is probably better, and on the other SQLite is nicer.

I worked on a project a few years ago, where I chose Firebird so I could use literally the same database on potentially offline sites that regularly sync up to a main office (shared) deployment. I worked pretty well and was still a lot of work.