Just got this in my email from BlackHatWorld:
Hi I just wanted to let everyone know that Google has just updated it's policy and the changes will cause some problems for publishers rotating ads from both AdSense and a competitive network, such as Yahoo Publisher Network.
"If you are running YPN and AdSense on a 50/50 ad rotation using the same or very similar color palettes, you would now be in violation of AdSense policies. Likewise, if you are running AdSense on one part of your site, and YPN on another part, you would now also be in violation of the policies if you are using the same or similar color palettes,". The rules also apply to ad units that mimic AdSense, yet are not contextual based.