Not sure what the implications of this are. Follow the links if you want to try to opt out of them.

Greetings from Google!

We're happy to announce that the Google content network now
accepts display ads served from qualified third-party vendors.
During this initial release, only ads in English are eligible,
although we look forward to offering more options in the future.

By accepting third-party ads, we can attract a greater variety of
advertising on the Google content network, which we believe will
result over time in increased revenue for publishers and more
relevant advertising for end users.

If you're currently opted in to image ads, you're already able to
receive third-party ads. If not, you can enable image ads to start
receiving third-party ads immediately. (Learn how to enable image
ads at
https://www.google.com/adsense/support/bin/answer.py?answer=9741.)

If you choose to allow third-party ads on your site, please update
your privacy policy to inform your visitors that third-party
vendors may serve ads on your site. Please also provide links to
these vendor websites and inform your users that they may opt out
of cookies (if the vendor offers this capability). For more
information about updating your policies, visit
https://www.google.com/adsense/support/bin/answer.py?answer=94150.

You'll continue to have full control over which ads appear on your
site with tools like competitive ad filtering and the Ad Review
Center. Also, only advertisers with whom we have proven
relationships and who've clearly demonstrated commitments to our
quality standards may participate in this program. Our policies
governing ad content and formatting remain unchanged.

To learn more about third-party ads, please visit our blog post at
http://adsense.blogspot.com/2008/05/introducing-third-party-ads-on-google.html
and our FAQ at
https://www.google.com/adsense/support/bin/topic.py?topic=14535.
If you have any questions or feedback, feel free to contact us at
adsense-support@google.com.

Sincerely,

The Google AdSense Team

Google Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043