Quote comes from this Engadget story, and you can also learn more on the Android Dev BlogGoogle is rightfully proud of its native theme in Ice Cream Sandwich. Dubbed Holo, it's intuitive, functional and -- dare we say it -- downright sexy. Now, in an effort to provide all users with a consistent interface, the company is making the theme essential for all Android 4.0 devices that come bundled with the Android Market. That's right, the stock, undiluted experience. Don't be alarmed, though, because manufacturer customizations aren't going anywhere, and the changes aren't terribly drastic. In fact, for users of Sense, TouchWiz and the like, Google is issuing architectural mandates that will make it dead simple for independent developers to create apps that fit nicely into these customized environments, complete with the proper widgets and colors. In other words, app designers may effortlessly choose between Holo or the manufacturer's default theme when considering the software's interface -- although, we'd really love to see this decision extended to end-users.
Quote from the Android Dev Blog:
So, now all the Android users don't have to look at Samsung's terrible TouchWiz UI or Sony Ericsson's....whatever that garbage is, and you have to admit, like Android or not, the new UI has some really good visual styles that maybe iOS 6 could learn from.In Android 4.0, Holo is different. We’ve made the inclusion of the unmodified Holo theme family a compatibility requirement for devices running Android 4.0 and forward. If the device has Android Market it will have the Holo themes as they were originally designed.
This standardization goes for all of the public Holo widget styles as well. The Widget.Holo styles will be stable from device to device, safe for use as parent styles for incremental customizations within your app.