Actually as we understand it now one of the ad services that displays banner ads was hacked. The ad service has been turned off. Until Matt can get Google to remove the GBO alert in google, if you use Mozilla Firefox you will continue seeing the warning.
Interesting enough, the warning from me was only coming from Mozilla Firefox and only when I "block attack sites" check in the security. When i went to IE, I got no warning. After looking around a bit this morning I found the firewall in my MacAffe Security suite was blocking the malware.
You can check and be sure with this malware search engine
http://www.malwarebytes.org/ Please pass this message to any else you think might be affected.