As mentioned in the KDE 3.1 Beta accouncement, KDE apparently has a set of User Interface Guidelines (à la Apple’s now-famous user-interface guidelines).
I’m pleased to see such a development in the Unix world. After all, it's not that I really need yet-another utility to parse text files ;), but I think that a healthy dose of UI rediscovery could do wonders towards the acceptance of Linux and other Unices.