I don't think we are yet at the point of the HTC UI being a threat. Not to undermind the importance of a good UI but for now it is only an interface and by itself not a significant thread to the WM space. UI frameworks that abstract the underlying operating system away are not a new thing (thinking of Java Swing, and
Qt).
IMO the bigger threat would be technologies that can be used to both provide a UI and envelope functionality such as Flash. As the OS becomes further abstracted it becomes easier to make a move from one OS to another while minimizine (if not eliminating) the amount of code that would need to be changed in an application to make it compatible with another platform
Microsoft has the ability to make such frameworks too (Silverlight for one, though portions of the .Net framework have been ported to other operating systems too). When Silverlight gets ported to Symbian and if it gets ported to Android then the playing field could change from being WM vs Andoid vs Symbian vs iPhone to Flash vs Silverlight vs iPhone for general purpose applications.