IMNSHO it should be done like this:
1) Get Zune to fully implement all publicly known MUI 4 functionality to achieve 100% source code compatibility.
2) Make Zune work on OS 3.x, OS4, AROS and MorphOS.
3) Once all that is achieved stop tracking MUI 4 and do whatever the feck the Zune developers like whilst keeping Zune working on all four platforms.