I second the Portable Allegroserve advice. It is an older project, but John Foderaro really put a lot of good functionality in Portable Allegroserve. That said, Edi's Hunchentoot project is very popular.
I wouldn't count on MySQL anymore. Oracle recently attempted to remove InnoDB support (i.e. transactional safety) from the Free Software variant of MySQL:
Although they took this back for the moment, allowing InnoDB to stay available in the "Community Edition", they made very clear that we can't count on that in the future.
So if you attach importance to ACID, I recommend to start with PostgreSQL rather than MySQL.
