The last stable version of the libgda and libgnomedb libraries have just been released. This new version brings a whole lot of new features which should make it easy for application developers to interface with several DBMS without any code modifications.
Libgda provides the low-level (non GUI) code to abstract DBMS access, while libgnomedb provides several data-bound widgets to interract with the data.
More information can be found at the reworked Gnome-Db web site http://www.gnome-db.org, and the source code can be downloaded from the usual GNOME FTP servers.