Recnetly there has been a noticable up-tick in the number of Virus or Malware cases for Mac's in the wild. Most prominently is the MAC Defender scareware that tells you that you're infected, and wants you to give up $40 for their software to remove the infections. Of course MAC Defender is the infection.
Fortunately, it's not too difficult to remove this little bug on your own. You can find removal instructions for the MAC Defender malware, as well as it's variant MacProtector on the Fix-KB website.
ZDNet has an interview with an AppleCare Rep that talks about how intense the up-tick is, as well as an article with sources from the Apple Support Forums.