Google content network now accepts display ads served from qualified third-party vendors. During this initial release, only ads in English are eligible, 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.
Just got the email from adsense support. I'm reproducing the email below for the benefit of those who are yet to receive it -------------------------------------------------------------------------------------------------- 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.
I think its ads from other contextual advertising programs with whom google has partnered. But yeh, the email is very vague.
Don't miss the original post on this subject at 10:06 - http://forums.digitalpoint.com/showthread.php?t=852512 Good stuff there.