if ($server == null && defined('DB_SERVER')) {
$dsn = ...
} else if ($server) {
$dsn = ....
}

During installation I ran into the case (not sure how!) where neither of these branches was executed. Net effect, $dsn wasn't set.

If I do a clean reinstall (source + MySQL + required db.xml hacks) and add an explicit $dsn = 'mysql://...' I can get it to work.

I guess this is not really a bug per se, but something's amiss.