After some silence it seems Apple is planning to release an update to Mac OS X specifically to remove MacDefender and to warn users of this malware.
I guess this makes it official now: there's malware for Mac OS X, and people will install it, with some good social engineering. It was just a question of volume and Mac OS X is probably arriving at that tipping point where it can be lucrative for scam artists to send out their software and try to get money out of people.