@shades et al
There is, simply, no significant threat for Macs. There are trojans... most of which hang out on porn sites pretending to be codecs. There ain't nothing, on _any_ site, that I want to see badly enough that I'd download a codec from j-random-site; I've got Perian, Apple's MPG-2, and Flip4Mac installed, if something needs a codec that's not in there I don't need to see it. There are other trojans which pretend to be installers for such things as various Adobe and Microsoft products; I either already have those products, installed from disc, or I don't want them, I'm not about to steal them. There are phishers; I don't click on 'click here' lines in email sent from places I don't have accounts with (Paypal, Bank of America, Barclays...) and if I do have an account there (MobileMe) I check the headers; I suspect, for example, that Apple is unlikely to be sending me something stating that my account is about to expire, in June, when I _know_ that it expires in December, and that Apple is _extremely_ unlikely to be sending out such notices using MSOE 6 from a server in St. Petersburg, Russia. And I have my accounts set up in bookmarks in a browser which does not use ActiveX and has scripting turned off except for the scripts _I_ want to run. If I want to go and check an account, I launch the browser, select the bookmark, do what I need to do, and then shut it down. I do _not_ 'just click' There aren't any exploits where a browser on a Mac (or on Ubuntu) can be hijacked merely by going to the wrong site; there are several where some guys try... and I get to see an .EXE file attempt to download in plain sight and utterly fail to install 'cause .EXEs can't run on OS X and I don't have WINE turned on in Ubuntu. And I have a proper firewall, including Little Snitch on Macs, which checks each and every outgoing TCP/IP connection and squeals if it's not approved. (Hiya, Adobe and Microsoft products, no you _CAN'T_ phone home, I won't let you except where, when, and how _I_ specify.) And I'm running as a standard account, not as admin, or, heaven forbid, root. The only way stuff gets installed is if it pops up a dialog and asks for my admin username and password... and I'm not entering that stuff unless _I_ started the install. Please note that all of this, except Little Snitch, is free, and Little Snitch is shareware and cheap. And I could use the free version if I was cheap, and willing to restart Little Snitch every three hours.
I haven't had a Mac virus on any system I'm responsible for in well over a decade (not difficult, the last serious Mac malware was the autostart worm from 1998 and that thing never got onto any of my machines 'cause I was running with proper security even in the days of OS 9...) and have not had a Windows virus beyond items emailed to me and detected and destroyed as soon as they arrived for almost as long.
But Mac users don't have a/v antibodies... Yeah. Right. A/v systems ain't coming onto _my_ Macs unless and until there is a real-world threat out in the wild which poses greater danger than the known drawbacks of a/v software. Little Snitch, for example, has used a whole 0.91 seconds of CPU time since I last rebooted this particular Mac, at around 12:35 Eastern time on Friday; it's now Monday, 15:50 Eastern. It currently is eating a whole 4.25 MB of RAM out of the 4 GB of RAM on this machine. And it doesn't do things like delete system files (or my email!) while it does do exactly what I want it to do, no more, no less. The day that a/v software can do that is the day I install it short of a real, verifiable, real-world threat. Which does not exist. Meanwhile, a/v software is even as I type eating people's system files...
If anyone wants to waste CPU cycles and RAM 'protecting' themselves against a non-existent threat, well, I can't stop them. (Hey, guys... if a real threat ever does appear for Macs, why do you think that current a/v systems will be able to detect it, and if detected, actually do anything about it? Wouldn't the a/v systems be completely clueless until someone gets hold of an example of that threat and can devise a fix for it?) I personally will not waste time, or especially money installing a/v on systems which are not threatened. If this makes me smug, so be it. I've been smug for well over a decade now and see no reason to change.