That's an interesting piece of a blog post at Google's Android Developers blog. Adam Powell, an Android Framework engineer who cares about style, explained in yesterday's posting what Google requires from its manufactures. With the release of Android 4.0, Google continued with the Holo system theme family which is further refined since its debut in Android 3.0. Before Android 4.0 the variance in system themes from device to device could make it difficult to design an app with a single predictable look and feel. Therefore, Google set out to improve this situation for the developer community in Ice Cream Sandwich and beyond and made the inclusion of the unmodified Holo theme family a compatibility requirement for devices running Android 4.0 and forward.
This means that if the device has Android Market installed, which makes it a Google experience phone, the device will have the Holo themes as they were originally designed by the Android team. However, while Google requires that Android experience phones have to include the Holo theme, it doesn't mean that Google goes to restrict manufacturers from building their own themed experiences (HTC Sense, Samsung TouchWiz, etc.) across their devices. But the "DeviceDefault" theme family and widget style family offer ways for developers to target the device's native theme with all customizations intact and even more important, the formal seperation of these theme families will also make future merges easier for manufacturers updating its devices to a new platform version; hopefully helping more devices update more quickly - at least it's Google's hope.
Anyway, while future Android 4.0 devices will still come with the manufacturers taste of design, it's quite interesting to know that manufacturers are not allowed/able to remove the original Android user interface anymore. Basically it means that devices could feature a user interface switch which allows to switch between Google's vanilla Android design and manufacturers own themed experience. However, since some original Android apps like E-Mail or calendar are also replaced with Sense or TouchWiz applications, it's most unlikely that manufacturers will offer this switching option. Nevertheless, it might allows smart developers to offer such a kind of "experience switcher" as a 3rd party application anyway.
Cheers ~ Arne