In my standard install of RoundCube 0.3.1, the remote client host is not logged to 'logs/errors'. It says (for example):
[17-Apr-2010 13:52:36 +0200]: IMAP Error: Authentication for <username> failed (LOGIN): "a001 NO Authentication failed." (POST /?_task=&_action=login)
Instead of trying to find a way to amend the log format, I have found it sufficient to use the roundcube-fail2ban plugin (by Matt Rude).