Well, the SEO poisoning attack I understand, but that is not what was being described. The way SEO poisoning works is that the search engine is tricked into generating a link to a malicious site rather than a legitimate site. This would be one of the normal channels that would bring up the bogus MacDefender web page that looks like a virus scan is being performed. You still have to click one of the bogus buttons (OK or Cancel) to start the download of the installer.
What was described was something different where clicking on the Google link didn't take you to the MacDefender ruse, but initiated the installer download instantly (or perhaps took you to the page and the download started automatically). I think the second option is not possible in Safari... but I could be wrong. I have seen no threads anywhere to suggest that I am wrong, however. I have my doubts about the first option, but am less sure. One of the posts indicated that it was tied to the Safari bug/feature where image files are automatically downloaded. Now I have never seen this behavior, but there are threads where people complain about it, so there must be some truth to it. Also, while I have never seen an image file get automatically downloaded to any of my Macs, I have seen something similar happen with PDF files, so that lends a little credibility.
I have one site that routinely downloads PDFs instead of displaying them in my browser, so I was able to try some things. When the PDFs were automatically downloaded, they did not open automatically open. I have to manually launch them. This behavior is independent of my setting of the "Open safe files" setting.
So, bottom line, I have my doubts that merely clicking on a Google search result can cause the MacDefender installer to launch. For that to happen, the Google search results link would have to be modified to trigger a download instead of following a link. I'm going to give Google the benefit of the doubt that their security has not been compromised to facilitate this.
I did say "doubts", which means I am still open to a convincing argument or proof...