martinibuster - 5:07 am on Apr 19, 2013 (gmt 0)
The email states there are two reasons for the update. The first is transparency. The second is related to mobile advertising.
The most important thing to note is that publishers are obligated to agree with these changes to the terms and conditions in order to continue with the program. So presumably, failure to log in to your account and agreeing to the change may result in getting booted from the AdSense program.
Publishers will be able to agree as of April 23rd, so be sure to log in and do that if you wish to continue with the AdSense program. You will have 30 days to do so.