SQLite (see last sentence under the "Serverless" heading) and BerkeleyDB should both allow concurrent access to the same store, however in the later case I think you do need to do some extra setup handwaving to ensure that locking is enabled (and I'm sure someone will chime in with a pointer to docs shortly).

Update: See also "Appropriate Uses For SQLite", which mentions that locking is done at the database file level (so only one reader will block out any number of writers, and likewise a single writer prevents any readers).