One major advantage to following this policy is that manufacturers will be able to update their handsets to the latest Android OS faster by keeping their skin apart from the OS it is running over. And by keeping menus and widgets all looking the same, it will mean that developers will have an easier time developing an app, knowing that it will look, feel and run the same on all Android 4.0 model smartphones. There will be three different Holo themes, Theme.Holo, Theme.Holo.Light, and Theme.Holo.Light.DarkActionBar.
It certainly sounds like the age of fragmentation is coming to an end for Android and Google has done a very wise thing by allowing faster updates along the platform while still allowing manufacturers to impose some form of individualization on their Android models. If you want to make sure that the Android phone in your hand is following the new policy, simply look for the Android Market. If it is on the phone, you know that the manufacturer is following the new rules.
source: AndroidDevelopers via AndroidCentrral