On WebmasterWorld, a member mentions that he received an email to upgrade his AdSense account to be handled under a Google login. He did not know if the email was legitimate because there was no official word from the AdSense site on whether it was.
Yesterday, on the Google AdSense blog, the details were confirmed:
That's right, we've just started our first Google Account migration tests for older AdSense accounts. When you log into your account, you may be greeted by a screen like the one below, asking you to upgrade your login to a "Google Account".
Since there was a lot of suspicion about the legitimacy of this email since there was no word from Google for 8 days, it would have been a little better for the peace of mind of all the users who received the email to actually have addressed this earlier, but it's good to have official word.
Forum discussion at WebmasterWorld.
