@David: we have the PEAR:DB integrated in the latest release and tested it with MySQL and SQLite. PostgreSQL still is buggy and the SQL queries have to be changed to work with it.
@Mark: I like your suggestion. If the whole data IO is written to work file based, it's easy to get it from the IMAP server instead of the local file system. The big benefit is, that you always have your prefs, contacts, filters, etc. available, even if changing to an other RoundCube server. Let's keep this idea in mind!
Thomas