According to the DB-Engines database ranking by popularity, the top 5 relational databases are: Oracle, MySQL, SQL Server, PostgreSQL, and DB2. In 4.6 we added support for SQL Server. The upcoming 4.8 adds support for PostgreSQL, thus completing Orbeon Forms support for the top-5 relational databases.
Past the top-5, we find Access and SQLite, which are typically not used in the type of server-side environments our customers have in place. The next candidate would be Sybase, but we're then getting pretty low on the popularity curve, and we've yet to encounter a customer who wouldn't be able to use Orbeon Forms without Sybase support. So it is with a certain sense of accomplishment that we're adding the support for PostgreSQL, thus allowing Orbeon Forms to reasonably claim it "supports, out-of-the-box, all the top relational databases".