Reports have uncovered a new variant of the Mac Defender virus/malware for Mac's, called Mac Guard. This seems to be an evolution of the Mac Defender, in much the same way many PC-based virus' shift and change to avoid detection and removal.
What this malware has evolved with is now the Mac Guard version doesn't ask you to authenticate administrator credentials, allowing it to feel less problematic when it starts it's install process.
While Apple is planning to release updates to OS X to help deal with this, I would recommend installing some sort of antivirus in the mean time. Sophos does offer a free home antivirus that will help with that.