“To use Google Ad Placement, you create and implement a custom channel just as you would before - tracking the performance of your sports pages or your skyscraper ad units, for example. Only now, you can choose to make that custom channel visible to advertisers as an Ad Placement. When advertisers search for sites on which to place their ads, they’ll see the ad placement you’ve created - with your site’s name and description - and can decide to target their ads to your Ad Placement.”
It looks like an extension of Targetable Custom Channels added a week or two back.
Here’s a screen shot of the new control panel in John’s AdSense back end.
Read John’s post at New Google Ad Placements