However, I am moderately jaded by the notion of usability defined as being "no learning required" - for example, which is more usable, vi or MS notepad? Anyone who has put in the effort would surely be able to use the former far more effectively than the latter, despite the difference in immediate accessibility.
It sort of muddies the water a little, I know, but I just don't think that 'ease-of-use' can be the last word in usability, and I think this is where free software excels - look at Firefox, for example. It has the instant gratification of any modern web-browser, yet offers almost emacs-level configurability.
I think that this is the happy medium that can only really come out of the free software world being, as it is, relatively immune to the sort of market forces that have forced the commercial sector to constantly chase the lowest common denominator. (who, let's face it, are bizarrely resistant to the idea that the incredibly complex piece of machinery in front of them might actually require some knowledge to use properly).
Ease of use doesn't equate "simple". Ease of use should mean that you can start using it with no learning curve, but that as your experience grows, you naturally learn to do more complex tasks.
The secret of ease of use is that for each level of expertise, from beginner to expert, the tool will seem intuitive, and also take you to the next level non-intrusively.
That's why it's so difficult to design right!
Then I discovered the dark side of business, money, marketing (and meetings, partnership negotiations, etc.).
You need both. But if you are an independent open source project with no backing from Sun, Intel or IBM, you are pretty much stuck with the "happy geek" problem.
Usability is one of those features that geeks tend to be most blind about. My favorite recent quote from such a geek: "the iPhone is nothing new, there are plenty of phones with the same specs".
Some people are trying to solve that problem as we speak you know ;-)