Android is no more open for handset manufacturers than Symbian, which has been around for years and is also open-source now. Even Windows Mobile phones have been seeing extensive customization lately - take the HTC TouchFLO phones, for example.
Honestly, it could be argued that Android is less 'open' than its predecessors, since there's an assumption that handset manufacturers will take advantage of the UI Android provides instead of rolling their own from scratch. I suspect we'll see much more UI consistency with Android than has been the case with the older operating systems.
As for users customizing their phones, there's nothing novel about that either. Palm and Windows Mobile phones have always let users install applications. In fact, you can easily run tethering applications on many Windows Mobile and Palm-based phones - just don't try it on the G1.
Honestly, it could be argued that Android is less 'open' than its predecessors, since there's an assumption that handset manufacturers will take advantage of the UI Android provides instead of rolling their own from scratch. I suspect we'll see much more UI consistency with Android than has been the case with the older operating systems.
As for users customizing their phones, there's nothing novel about that either. Palm and Windows Mobile phones have always let users install applications. In fact, you can easily run tethering applications on many Windows Mobile and Palm-based phones - just don't try it on the G1.