I think the companies that write anti-virus software are just a little upset with the Mac in general seeing as it's a bit difficult (short of deception) to sell "anti-virus" software for a system that to date (OSX anyway) hasn't had a single virus. Not only would you not need a daily update of the virus list, but you wouldn't even need the software period since it serves no purpose (unless you enjoy filtering Windows viruses out of e-mail for your friends...assuming you forward every stupid thing every sent out known to man without even looking at it).
I know a lot of people say the Mac and Linux don't get viruses because their market is too small to bother, but I say that reasoning is a load of bologna. The Commodore Amiga never had ANYWHERE NEAR the current user base of OSX and it had a LOT of viruses created for it. Admittedly most were floppy boot block viruses and weren't all that lethal, but that doesn't change the fact they were viruses and they were on a relatively small platform.
OSX and Linux have one other thing in common and that's their Unix (or Unix-like in Linux's case) heritage. It seems like it would be hard to comment on the susceptibility of a Unix system to viruses if none have ever been made for it. One could suppose it would be still be easy to do, but until someone does it, the proof seems lacking, IMO. Trojans and worms aren't the same thing, BTW.
It just seems like the Mac is a prime green field of completely un-mined, unsuspecting, completely vulnerable machines with absolutely no anti-virus protection. Come on. You're telling me 10+ MILLION unprotected machines wouldn't interest a few criminals while there are literally hundreds of thousands of viruses for Windows? Even by percentages this makes no sense. There should at least be a couple hundred or so Mac viruses out there. There are NONE. Susceptible to hacking? Yes, that's been demonstrated. I haven't seen a virus demonstrated (even in a security contest type thing) yet for the platform. I don't WANT to see one appear (given it's one of the biggest reasons I use a Mac on a daily basis instead of my Windows machine), but I don't buy the "small user base" theory alone. It has to be harder than just that or we'd at least have a few by now.