This showed up in my mailbox:
Dear Publisher,

We noticed that a portion of your AdSense revenue is coming from an older version of the AdSense for search box on your pages. On February 15, 2011, we'll retire this early version of the AdSense for search and replace it with the more powerful Custom Search Engine (CSE).

If you're not sure which version of AdSense for search you're using, check the Google logo on your search results page. If it reads "Powered by Google" instead of the newer "Google Custom Search" then you haven't yet upgraded to the newer version.

To ensure you don't miss out on any revenue during the transition, we encourage you to update your code now. If you are using the old AdSense interface, follow these instructions to regenerate your code. If you are using the new AdSense interface, make sure to follow these instructions.

If you'd like to access advanced features of the Custom Search Engine product like refinements and promotions, you can access all advanced features at Google Custom Search - Site search and more. To learn more about the benefits of CSE, read our blog post here.

If you have any questions, please visit our help center or ask a question in our support forums.

We appreciate your understanding and your continued participation in AdSense.

Thank you,

Your Google AdSense Team