Apple released Mac OS X Lion developer preview 4 ahead of the launch in July through the App Store. As expected, not many of the changes have been noticed from many people except a revamp in the login screen. You can check out the screenshots of both the login screens (old and new) and realize the difference between them. Earlier, it used to be a light grayish feel of the screen which has been now changed to the Dark grey.
New Login
Old Login