WebmasterWorld members are reporting receiving an email from Google that they will be launching an "Ad Review Center."
The Ad Review Center will be launched within the next few months and enable AdSense publishers the ability to approve ads that are submitted for their site via the placement-targeted method.
Google launched placement targeting in September 2005 and then started displaying Advertise on this Site on those ads that support that feature. Google told me they will be dropping the "advertise on this site" from the AdSense ads.
So this new feature will give publishers even more control on the ads that are displayed on their site. Of course, an advertiser can block any ad using the competitive ad filter. But this feature seems like it will allow publishers the ability to approve the ad before it even goes up.
Forum discussion at WebmasterWorld.