One of the numerous promises Google made with the release of Android 4.0 Ice Cream Sandwich was that something was going to be done about fragmentation. It took them a while, but finally they has solved one of the major fragmentation problems with Android: the numerous custom UIs. 

As of now, any device that wants to have a Google approved installation will have to include Holo, the unmodified default theme of Ice Cream Sandwich. In case you don't know, what gives this policy teeth is the fact that only Google approved devices get access to the Android Market and proprietary Google applications like Gmail, Maps, and even Google Search.

Considering any flagship device would be sorely lacking without these things, I think it is safe to assume that all of the major manufacturers will comply with the policy. This will not only be good for consumers, who will then be able choose to use the stock UI at any time, but for app developers as well, because they will be able to add much more consistent user interface elements to their designs.

While this policy isn't quite as good as giving consumers stock Android and allowing them to install the custom UIs, it is quite similar to that solution which I proposed nearly a year ago. It is great to finally see Google finally fighting to stop fragmentation and keep Android open for the consumer, but they still have a way to go. For example, with this out of the way they might want to start working on a system to ensure reliable updates.

[Android Developers Blog]