Pam Parker's: Google Tests 'Advertise' Link, Landing Page, reports on another test at Google. This time, viewers of Google advertising on "select" Google Network sites will see the text link, "Advertise on this site" next to the AdSense ads.
If a user clicks the link, they're taken to a Google hosted landing page where they can learn about purchasing site-targeted advertising on that specific site. Another link on the landing page takes the user to an AdWords sign-up page. More about the test along with the usual "we're always testing things" comment from Google in this Clickz article and this page from Google that explains the "advertise on this site" page.
Postscript: AskDaveTaylor has more including a screen cap of the landing page.