Nice article on the apple blog - More Mac Viruses, Similar Sources: Time to Worry?
Really, as it stands, the only people at risk are those trying to pirate software, so it’s not really a case of “Is the OS less secure?”, so much as it is one of “Are Mac users security savvy?”.
Personally I look at it this way... if you download the iwork trial from apple.com and install it, it'll ask you for your password before it installs itself. If you go to your friendly neighborhood torrent site, download ilife09.torrent, and click the installer it (I presume) asks for your password before it installs a trojan and then installs iLife.
Is this some new form of super-evil mac virus? An age where apple-heads can't surf in peace? An age where you shouldn't suggest an iMac to your grandma because it'll come to life, kick her in the behind, and impregnate her granddaughter?
I'm thinking not. I could put up a shell script that has in it a request for password followed by deleting your system while it plays a clown song and put it up for download and most likely it'd still get downloaded and accused of being an evil mac virus. The sort of viruses we're talking about here are simple trojans (IMHO) and requires no more than the normal (well, perhaps normal by windows and I-surf-pirate-sites standards) amount of care and attention in surfing and downloading.