Last night, Google announced they are opening up the "Google-certified ad networks" to all AdSense publishers. The Google-certified ad networks are 3rd party advertising networks that are thrown into the bidding war with your AdSense powered ads.
Google initially tested this back in September 2009 and in October publishers began thinking that it lowered their revenue with Google. So some publishers decided to block these ads. Google then came out and told publishers not to block them because it was only a limited test and the lower earnings likely had nothing to do with the 3rd party network.
Now, it is available to all publishers, so publishers can test it better. As Google notes, you have control over it in the ad review center. So give it a try and if it doesn't work for you, kill it.
Forum discussion at DigitalPoint Forums.