Google AdSense has a new homepage that looks the same as Google AdWords homepage and offers more information about the service.
The old homepage, still available here:
The new homepage:
The redesign also reflects that AdSense started to migrate users to Google Accounts. "When you log into your account, you may be greeted by a screen (...) asking you to upgrade your login to a Google Account. If you like the email address you're already using, in most cases you'll be able to upgrade it to a Google Account and keep the same login and password. However, if you use another email address for other Google products like Gmail and Picasaweb, you already have a Google Account and you can finally use the same login for AdSense."
Google AdSense was launched almost four years ago, when Google didn't have an integrated authentication system and most of the services didn't require registration.