Gunter Ollmann posted an excellent article explaining SEO Code Injection attacks at http://technicalinfo.net/papers/SEOCodeInjection.html. This is one of the best explanations of the attack I've read. Go read it. NOW!
SEO code injection attacks have been gaining popularity by those evil malware authors as a way to get unsuspecting victims to their attack pages. A few highly publicized attacks were done earlier this year which resulted in alot of head-aches for some major sites. Dancho Danchev has alot of excellent information on these attacks on his blog.