From Mac Observer: If you have installed OSX 10.6 latest Security Update 2011-003, you will get daily safe file updates for Safari to prevent malware (trojan horse) from infecting your system. This is a direct response to the MacDefender malware back in May this year.
Unfortunately, there is no guarantee that this works perfectly and you get the latest virus definition every time one is available. Thanks to Adam Christianson from Mac Observer, he had updated Safe Download Version to have the ability to grab the latest definition list directly. This OSX utility also shows you when the definition list was last updated and what version number is installed.
Safe Download Version is free and it is available for download on the bottom of TMO page here.