We all need to use anti-virus, but above all we need to use our common sense...
The Flashback Trojan is a good example of how Mac users are now very much targets of cybercriminal gangs - no longer are these people a ragtag bunch of hackers simply looking to make a name for themselves but well-organised, profit-motivated gangsters - as was the MacDefender (and numerous variants) scareware campaign from last year.
Don't be fooled into thinking that Apple necessarily has your back either - it seriously dropped the ball when it came to the Flashback threat, not issuing an update for Java for weeks after Oracle had made it clear that a vulnerability existed and had fixed the threat for Windows.