According to the BBC, Google has started to warn users when they are about to click through to a site which contains malware.
This raises two questions:
Why include ‘bad’ sites in the listings at all? It is already part of their quality guidelines that a site must not contain viruses, trojans or ‘badware’. They already remove or penalise sites for infringing on other guidelines (and occasionally for no reason whatsoever), so why not enforce this one as well?
How will this affect Google Adwords? Going purely by SiteAdvisor, the majority of the sponsored links for the ‘dangerous keywords’ given by the BBC are rated as ‘Use caution’. If Google was really acting with the searcher’s best interests in mind, they wouldn’t accept advertising from sites which attempt to either scam a user by charging a subscription for products which are available for free, or spread malware. But of course, they couldn’t possibly do that, it might hurt their bottom line.
In my admittedly short testing period, I could only find one site, which I had to search for by domain, which triggered this doorway. The SiteAdvisor plugin however listed several bad sites for each search, as well as a link to why the site was marked as such.
If you are an IE or Firefox user on Windows, I highly recommend the SiteAdvisor plugin from McAfee. (Probably the first and only time I will recommend a piece of McAfee software. 
) It has a much more thorough collection of sites, warns of spam and scam sites as well as malware, and checks every site that you visit. If you are not paying attention, you might miss the SiteAdvisor warning, but it also alerts you if you attempt to download anything from the site.
I applaud Google for taking these initial steps against malware, but if they want to do so, they should do so properly. Integrating something similar to SiteAdvisor into the Google Toolbar would be a good start.