Error message when trying to run scheduled email send - Joomla
I don't think the problem is what you think it is (on line 28) because otherwise you should get that same notice (and fatal error) on line 29.

There are a whole bunch of bugs submitted on issues.civicrm.org relating to cron in early versions of 4.1.  Since CiviCRM 4.2 supports Joomla 1.7, my advice is to upgrade to CiviCRM 4.2.20.  Civi 4.2 was a "long term support" release, which means it was getting (some) fixes through April of this year.  It should be in far better shape (especially as regards the cron system) than Civi 4.1.2.