It seems using other adverts that look similar is out, and Jennifer goes into detail how that can affect split testing, or having adverts from different sources in different parts of your site.
In regards to competitive ad units, this is what the Google guidelines now say
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.
Google have taken some stuff away, but I think they might have given something in the other direction.
Maybe I have been out in the woods for too long and missed a previous change, but from what I understand having just read the document, you can now use Chitika in a totally contextual manner along side Adsense.
Chitika look nothing like Google Adsense
It is possible to automatically add keywords to Chitika adverts on your WordPress blog, but honestly it is hard to determine which keywords result in higher paying adverts.
It is much better to allow Chitika to control advert inventory. Hopefully that way they can automatically tweak performance for the best adverts, just like Adsense.
Update: Darren has now posted about the Adsense changes, but I don’t think he spotted that you might be able to use contextual Chitika now.
Update2: See comments for Darren’s note about the terms of service, and the questions I have asked the Adsense team.
Disclaimer – It is your Adsense account – consult with the Google Adsense team if you are unsure about any terms which you have to agree to display Adsense on your websites