I reported earlier this morning that some scam artists are using DNS Cache Poisoning to redirect your PPC and organic listings to other pages. First reports of this come from WebmasterWorld, where it has been documented that an advertiser was targeted and exploited. Basically, the attacker will poison your DNS cache (if it is possible) and then “redirect a popular search engine to a pop-up ad site,” or something similar.
SEO Consultants has a very detailed write up on this vulnerability. You should immediately check your domain names at DNS Reports to ensure you pass for “Open DNS servers” test.