I read a couple of interesting blog posts today about sites getting hacked.
Sunbelt Blog had an example of a hacked site, where the site redirected you to malware if you got to the site through a link (such as from a search engine). Otherwise the site displayed normally.
The Kaspersky Analyst diary had more information. In a dark form of search engine optimization, the attackers would find search results for a search term, and then compromise the popular results that they could. Adding an iframe is so 2006, so they’d modify existing javascript on the page to run their code and redirect users to Antivirus 2009 websites.