Google is working to curb that, and recently has purged a mass number of malware sites from their indexes. This is something that I think all search engines would want to go anyway, as obviously a site hosting malware is probably not going to be high up on the relevance list of someone looking for something. While it is good to see Google doing this, the article mentions and many others could speculate that this should be something looked into more often. Purging sites from search indexes won't stop malicious emails, but it's a good building block to helping solve a larger problem.
The only downside I see to this is a potential for innocent sights to get caught up in the purging. We've heard of situations like this before, with Internet Explorer 7 identifying a site as compromised or malicious when it is not. Still, the benefits likely outweigh the downsides and I hope Google and other search engines take this even further.