if you wish to install by yourself RoundCube 0.6 on an IspConfig 2 server, after you downloaded, unzipped and installed RoundCube (or upgraded from version < 0.6) remember to perform the next modifications on the file config/main.inc.php
1) Modify the line:
$rcmail_config['plugins'] = array('new_user_identity', 'virtuser_file');
(Before RoundCube 0.6, the 'virtuser_file' plugin didn't have to be activated explicitly)
2) Add the line:
$rcmail_config['virtuser_file'] = '/etc/postfix/virtusertable';
(/etc/postfix/virtusertable is kept up by IspConfig2)
I hope this can help other people to save time during RoundCube setup.