The best way to learn about major open source databases (e.g. MySQL, PostgreSQL, etc.) is to attend developer and user conferences. Sun Microsystems sponsors many of these conferences and events. (This April, you can catch Sun folks attending the MySQL conference in Santa Clara, and in May, you can catch them at PGCon in Ottawa.) Finally, if you're interested in Sun technologies and databases, you should become a member of the OpenSolaris Databases Community and start contributing.