Many search engines, such as Yahoo and Google, cache entries over time. There are many benefits to this, but also a serious drawback. It seems that search engines are not purging malicious sites from their caches, which can result in people still easily finding sites by pulling up the cached version. Cached sites eventually expire, but some sites keep archives for many years.
Security firm Aladdin has brought this to the attention of many, pleading for the search engines to do something. While it is hard to point a finger at a search engine for helping spread malware around the Internet, it would seem that they would be capable of doing some form of cache screening or purging that would look for malicious code. Then again, that would be a monumental task. It was a problem years ago, and remains one today.