We've been warning you to link your Google AdSense account to a Google account for months. Google is now making it official. A WebmasterWorld member says that he received an email saying that he will have to do it soon or else. His reason for not doing it beforehand is that it wasn't broken and it's just kind of pointless for Google to fix what ain't broke.
Meanwhile, in our previously mentioned Google Groups thread, a few updates and clarifications have been made. First, Ashley from Google says that once your account is migrated, you will have to use that Google email address to log into your AdSense account. Second, she says that if you have another Google account that is not a login for your AdSense account, it can be deleted without disabling access to the AdSense account (but it will disable access to other services associated with that login).
Of note: one member is having difficulty performing the actual merge. Maybe it will still be a few weeks while Google irons out the technical problems with this. The discussion related to this technical issue is at Google Groups and additional forum discussion continues at WebmasterWorld.