Jennifer Slegg informs us that Google has added new custom targeting channels for AdSense publishers. The new option allows AdSense users to label different ad units on their site, and allows AdWords advertisers to see this units and select them for their ads.
Publishers will need to turn the feature on for each relevant custom channel in their account, or set up brand new custom channels, breaking down the site or ad units in ways they feel advertisers will want to target them. Then advertisers can access the channel information for specific publishers in order to pick and chose what to target on the site.
Jen has more over her own blog.