Google has announced that the Holo theme family is required for devices running Android 4.0 although manufacturers will also be able to build custom themes.
Specifically, Google has:
made the inclusion of the unmodified Holo theme family a compatibility requirement for devices running Android 4.0 and forward.
The "Holo Everywhere" announcement was made on the Android Developers Blog and seems to be an effort on Google's part to introduce some uniformity across different devices and go part way to addressing aspects of the fragmentation that we so often complain about and is presented as a benefit to developers and end users alike.
According to Adam Powell,
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. We set out to improve this situation for the developer community in Ice Cream Sandwich and beyond.
If the change helps more devices update more quickly, the key benefit outlined in the announcement, it will indeed be beneficial.
Let's hope that the device manufacturers also get the message and don't defeat the system in the attempt to continue to distinguish their particular phones by using ever more non-standard skins. The new requirements only insist that the manufacturers include the Holo theme. If they don't then their device won't be an official Android 4 device and it will be denied access to the app market.
Having the Holo theme available is going to make uniformity more possible, but it is up to the app developer to select either the standard look or blend in with the rest of the phone's custom look - this could be a difficult choice.