From now on, any Android 4.0-powered device that comes baked in with Android Market, will sport the "Holo" theme. In an effort to provide all users with a consistent interface, Google is making the move.
Holo does look impressive, and is one of the best UI's I've ever seen (in videos, pictures) and I can't wait to slap it onto my Samsung Galaxy S II. However, manufacturer customizations aren't disappearing. Users of Sense, TouchWiz, etc should still all be A-OK. Google are issuing architectural mandates that will make it quite simple for independent developers to create apps that slide perfectly into the customized environments, complete w ith the proper widgets, and colors.
From now on, app designers can choose between Android 4.0's Holo theme, or the manufacturers default theme when considering the software's interface. I like the decision from Google, as it brings them more in line with Apple and their one-and-only theme (unless jailbroken).