Consider yourselves warned (NYT article below). MAC DEFENDER IS NOT YOUR FRIEND.
If you want a guide to antivirus programs for the mac, Lifehacker has a good one.
Apple computer users were targeted by fake antivirus software that hopes to gain access to users' credit cards. Apple said Wednesday that it plans to issue a software update to remove the malware.
Update: John has found this article on Apple.com about how to remove Mac Defender if you did end up installing it.