An oft-requested feature has been implemented in CodeIgniter’s session class. If you use a database to store your session info, then all user data will now be in the database, rather then an encrypted cookie. From the subversion repository’s changelog
Updated the Sessions class so that any custom data being saved gets stored to a database rather than the session cookie (assuming you are using a database to store session data), permitting much more data to be saved.
More details available in the manual. Please read the svn upgrading instructions, as a new field needs to be added to the database for this.