Manufacturers are still free to customize to their heart's desire. All this means is that the rom manufacturers use must have the holo widget styles included so third party developers can use it in their apps without interference. That way, when they design their app around the default 4.0 UI, they'll know it will be consistent across devices.
yes, but hopefully the API around themeing divorces the carrier customization from the working of android itself. before ICS themeing was actually modifying android, not they can build a theme and apply it from default without actually having to tinker under the hood. Hopefully this allows faster updates to products that use manufacturer themes