Like skiers, the average user is a permanent intermediate, neither beginner nor expert. It is a mistake for UI designers to heed debates such as going on in Scoble's comments on UI requests for Longhorn. Even more dangerously, intermediate skills are easy to overlook in usability labs: sit a person down in front of a new application and they will act like a skier on their first run of the new year -- either extremely tentative and "beginner-like" or bold and "expert-like."
But after the first couple sessions with an application, people perform particular tasks with facility. Good UI design does not force the user to categorize themselves as "beginner," "intermediate," or "expert" (as Richard Tallent says, this argues against layered UIs). The key to great UI design is to facilitate the skills of routine tasks to the demands of new tasks: look to videogames for inspiration in the way that a small number of "skills" can be combined to tackle complex tasks.
I first heard the "skier" metaphor from Larry Constantine and Lucy Lockwood.