@Sally Stitts -- Great question. I'm a Google Product Manager working on this launch. Here's a few quick clarifications that hopefully address your question:
The only ad units affected by this launch are AdSense for Mobile Content ads using the "iPhones and other highend devices only" setting. These ad codes will need to be updated by May 1, 2012.
-- For an illustrated explanation, have a look at this screenshot: [goo.gl
-- If you are not using the AdSense for Mobile Content product, you won't be affected.
-- If you are using AdSense for mobile content, but your ad units use the "All Phones" setting, you won't be affected.
For more details, see an excerpt from the notification email that went out to publishers.
-------- Excerpt from email to AdSense publishers --------
To complete this migration, we're asking all publishers who have AdSense for mobile ad units that are meant for "iPhones and other highend devices only" to update their ad code as soon as possible. As of today, you will find the mobile ad unit sizes within the AdSense for content section of your AdSense interface. Implementing Google AdSense now will allow you to continue monetizing your mobile content after May 1, 2012 without disruption.
If you are using the AdSense for mobile ad units meant for "All Phones", you will continue to be able to serve ads to both WAP phones and highend phones.
For more questions about how our mobile ads products will work now, click here:http://goo.gl/BtTcI. For additional support on Google AdSense, visit the Help Center at [support.google.com
Hope that helps,