EFF  Simply because you've never encountered one, doesn't mean they didn't exist. I caught one back in days of my trusty ole' SE30 while installing a game from a floppy I had purchased at a local book store. Luckily, it was a variant of Code 252 which just displayed a creepy message and then deleted itself.

And if you don't believe the page I linked to above, take it from the horse's mouth:

http://docs.info.apple.com/article.html?artnum=50569

http://docs.info.apple.com/article.html?artnum=30345 (under "MBDF virus and Mac OS 8")

So yes, you are correct in that there are currently no viruses for any version of OS X, and with very few exceptions OS X users don't need to be concerned about the issue (I don't have AV software installed on any of Macs, btw), aside from exercising common sense when surfing/downloading files.

But that does not mean that a) there never were any viruses on the Mac platform in the past, and b) there may not be poo-ware attacking OS X in the future.

All that being said, I think we can agree on two points: Norton AV, in its current state, is a POS, and the AV developer's money-grabbing schemes are ridiculous.  
(Version 9/24/09)

ILGAZ  If this release included "signature updates for the unpatched java security hole on pre OS X 10.5 systems" as release note, I would run to Symantec store and buy it.

Of course, they are so busy with including Windows virus signatures and proof of concept junk rather than real threats, backdoors.

I don't want to spam this site so consider this comment for every single AV running under OS X, even including "free" ones. If you aren't protecting against an active threat, you don't deserve a single CPU cycle.  
(Version 9/4/09)

CYBORG9  Norton antivirus? The mere mention of that name should give any self respecting Mac user a case of spontaneous, explosive diarrhea.

Truthfully, as has oft been said, there just ain't no active virus threat out there in the wild for Macs at the moment.

One of these days, there likely will be.

So, it might be wise to have some anti virus protection at one's disposal. There is, though,

no reason to pay through the nose for some virus protection such as the dreaded and sickening Norton or the quite overpriced Intego VirusBarrier (they've got a $30 yearly fee for virus definition updates after one's first year of Virusbarrier use).

There's the highly regarded and free ClamX.   
(Version 9/1/09)

FRANKHOGG  I've been using Macs since 1984 and I don't use any "AntiVirus" software, free or paid for simply because THERE ARE NO VIRUSES FOR A MAC! Never have been. I've never had a problem... NEVER!

Do you know why there are two lion statues in front of the NY Public Library? The answer is they keep the elephants away. Hey it works just as well as any anti virus software for the Mac does.  
(Version 8/18/09)

ARINSBLOGCDYT  Just another happy user of ClamAV. I would say that unless you really hate your machine, stay away from the bloated NAV. I have had enough of it. Can you imagine I discarded it even after buying it for a decent amount! That is because I love my computer just too much to torture it with Norton.

Ever since I installed Clam, my machine has become much faster and on top of that, it is yet to get affected with a virus or trojan!

I also have a machine that runs on Windows XP and I use the free Avira on that one!  
(Version 8/07/09)

SHAWNABECKLAR  Been looking for this update. So this makes it easy. thanks guys  
(Version 8/04/09)

SCOTT_E_COLLINS  Norton Antivirus feels somewhat to 'bloated' for me. I personally prefer ClamAVX. Also heard good things about McAfee, but ClamAVX seems to have the most clean, no-nonsense 'feel' to it.  
(Version 7/15/09)

RMENDES  with all the updates this is getting, it must include the h1n1 anti-virus as well...  
(Version 7/14/09)

26CAB40  This udpate is very poor.

It does not improve Norton, an already poor application.

I suggest ClamXAV as a better alternative.  
(Version 7/14/09)

MARK EVERITT  Is all discussion post-2006 simply being deleted? I know it's been heated lately but some good points were made.

These virus definitions can only be used with the non-free Norton AntiVirus. I'd say that this does not fit into any meaningful definition of free.

Secondly, these definitions seem to be supplied separately because the automatic updating in NAV does not function under 10.5.7 (looking at the comments for NAV). Considering that Symantec deals by keeping us scared of our own shadows I'm going to be consistent to that and put it to them that using a separate installer to update definitions is an enormous gap in their protection. If automatic updating does work, then posting the definitions here is shameless (and frankly bewildering) advertising.

This is a shocking state of affairs.

For the record I run ClamAV, not because I fear viruses but because I work with some windows users. If they get a virus then it slows me down too (I've tried to convert them to Mac or Linux to little avail). The main free competitor to Norton on a mac is ClamXav, which has no problem with updating itself.  
(Version 7/14/09)