John Gruber has a depressingly good article about Linux usability in which he highlights some of he fundamentally wrong assumptions about usability, and, in particular, why Linux is (can be) terrible in that department. The underlying assumption is that first you make software work, and then you “spray on” the ease of use.
In the article he refers to the “rock star” UI designers who are both good at user interface design, and, at the same time, are programmers and can make it happen. He says that these folks are very very thin on the ground. Of course, I immediately thought of Tim.