Apple has released a knowledge base article titled "How to avoid or remove Mac Defender malware" regarding the Mac Defender scam. It obviously explains how to remove it if you already installed it and how to avoid it. But it also says:
In the coming days, Apple will deliver a Mac OS X software update that will automatically find and remove Mac Defender malware and its known variants. The update will also help protect users by providing an explicit warning if they download this malware.
Good. Apple needs to keep an eye on the ball when it comes to these issues. It's a big differentiator.