According to Droid Life and the Android developers blog the Holo theme included in Android 4.0 is a requirement on a devices that ship the Android Marketplace with 4.0. This promises to fix is the amount of time it takes manufacturers to produce updates to the OS. They do this by making Holo a requirement and allowing a DeviceDefault skin as well.
Adam Powell of Google says:
Formally separating these theme families will also make future merges easier for manufacturers updating to a new platform version, helping more devices update more quickly.
Also, developers can specify DeviceDefault or Holo. Since every device going forward will have Holo, this can help provide a more consistent experience across devices.
From the final thoughts on the Android Developer blog:
Android apps running on 4.0 and forward can use the Holo themes and be assured that their look and feel will not change when running on a device with a custom skin. Apps that wish to use the device’s default styling can do so using the DeviceDefault themes that are now in the public API. These changes let you spend more time on your design and less time worrying about what will be different from one device to another. Finally, Android’s resource system allows you to support features from the latest platform version while offering graceful fallback on older devices.
I think this is something Android has needed for a long time and it will only help fight the “fragmentation” problem. It’s not a 100 percent solution, but it’s one that is a big step in the right direction.