Most probably you have already heard about last Google Play Developer Program policy changes. There is a lot of rumor on the Internet. Most blogs and News site says about the end of Notification Ads as new policy says:
Ads must not simulate or impersonate system notifications or warnings.
It was my first thought as well. The end is coming. But some people on the Android forums and some mails I have exchanged made me wonder about intention of Google. Did they written that notification Ads are forbidden or...? Lets look on the content policy a little closer.