I'm running the 20051021 build on PHP 5 and MySQLi. Working fine so far. I do not have a lot of people to test it. I had to make the following changes.
Insert the following after the mysql case ending on line 174 of rcube_db.inc
case 'mysqli': return mysqli_insert_id($this->db_handle->connection);
In db.inc.php make this change to line 21
$rcmail_config['db_dsnw'] = 'mysqli://user:pass@localhost/dbname';
If you have chrooted mysql and have to use TCP like me then remember to change the host accordingly.
$rcmail_config['db_dsnw'] = 'mysqli://user:pass@127.0.0.1/dbname';
If this was already posted by somebody then never mind. :-)
Adam