Apple now suggests "multiple" Antivirus programs

After years of bragging about MacOS X's invulnerability to computer virii (there has never been a virus for MacOS X seen in the wild), they are now changing their tune. Not only are they recommending antivirus software, but actually recommending multiple, overlapping software so that hackers can not target only a single algorithm. This leads to the obvious question: "What do they know that we do not?" Is this simply a move to insulate them against potential future lawsuits, or is there a MacOS X virus out there that we just don't know about yet?