According to Android developer Jared Rummler and a number of other sources, Google has begun purging known ad-blocking software from the Play Store today.

Got a notification from Google and it looks like all Ad Blockers were removed from Google Play today. — Jared Rummler (@jrummy16) March 13, 2013

Popular app AdAway was among those removed, and received notice that its app was in violation of section 4.4 of the Play Store Developer Distribution Agreement. Here is the cited section:

4.4 Prohibited Actions. You agree that you will not engage in any activity with the Market, including the development or distribution of Products, that interferes with, disrupts, damages, or accesses in an unauthorized manner the devices, servers, networks, or other properties or services of any third party including, but not limited to, Android users, Google or any mobile network operator. You may not use customer information obtained from the Market to sell or distribute Products outside of the Market.

There really isn't much wiggle room on interpretation here - ad blockers clearly do interfere with the properties and services of third parties (including Google, which is probably a big one not to interfere with). Most of the popular ad-blocking tools seem to have vanished, including Adblock Plus, which recently fell victim to a bug fix in Android that made the service much more difficult to use. Another very widely-downloaded solution, AdFree Android, is also gone.

AdAway had this to say about the message they received, and advised users to switch to an open alternative:

AdAway was removed from Google Play I received an email from Google Play that AdAway was removed due to "Violation of section 4.4 of the Developer Distribution Agreement." Thus, it will never be available on Google Play again. Please switch to the open alternative: F-Droid (https://f-droid.org/FDroid.apk) and install AdAway with F-Droid.

Here's the message Ad Blocker received regarding the takedown:

This is a notification that your application, Ad Blocker ★ Root ★, with package ID com.jrummy.apps.ad.blocker, has been removed from the Google Play Store. REASON FOR REMOVAL: Violation of section 4.4 of the Developer Distribution Agreement. After a regular review we have determined that your app interferes with or accesses another service or product in an unauthorized manner. This violates the provision of your agreement with Google referred to above. All violations are tracked. Serious or repeated violations of any nature will result in the termination of your developer account, and investigation and possible termination of related Google accounts. If your account is terminated, payments will cease and Google may recover the proceeds of any past sales and/or the cost of any associated fees (such as chargebacks and transaction fees) from you. If your developer account is still in good standing, you may revise and upload a new instance of the application that is compliant with the developer terms. Before uploading any new applications, please review the Developer Distribution Agreement and Content Policy. If you feel we have made this determination in error, you can visit this Google Play Help Center article for additional information regarding this removal. The Google Play Team

Some of these apps are still available as cache on the Play Store, but all have been removed the affected developers' app listings. We'll update this story as we learn more.

The app was not found in the store. :-( Go to store Google websearch

The app was not found in the store. :-( Go to store Google websearch