This is a pretty shocking announcement, since in the past Apple has used the virus issue as one of their selling points in their snarky, smug, "I'm a Mac, I'm a PC" commercials.

Apple to Mac users: No anti-virus software? Get it now!

Apple encourages the widespread use of multiple anti-virus utilities so that virus programmers have more than one application to circumvent, thus making the whole virus writing process more difficult.
And even more interesting.... Apple PULLED THE STORY off their own web site.

Apple pulls support note recommending antivirus software