Adsense updated their TOS [Terms Of Service] with a few changes that shows how much they hate competition!
Competitive Ads and Services In order to prevent user confusion, we do not permit Google ads or search boxes to be published on websites that also contain other ads or services formatted to use the same layout and colors as the Google ads or search boxes on that site. Although you may sell ads directly on your site, it is your responsibility to ensure these ads cannot be confused with Google ads.
What does this imply for thousands to publishers all over the globe? Simply put, you cannot run ads of any other network which look like Adsense ads. If you do, you are violating the TOS. Either you forget all other PPC networks, or forget the Adsense Revenue. Which way is easier? Another alternative is to change to looks of other ads so that they don’t even remotely resemble the Adsense.
My 2 Cents:
I thought Adsense was way ahead in the market competition and that they should be concentrating on building more value than be fighting the other PPC networks.