Design Sessions has a good interview with Luke Wroblewski. In particular, I thought the following passages were very interesting:
"Q: How do you know when a design is successful? How does your definition of success relate to usability, technology, and art?
Luke: A site is successful when it achieves the goals you set for it. I know that's a simplistic way of looking at things, but there are so many considerations that impact user experience: findability, usability, desirability, accessibility, credibility, technology, etc. Focusing on any one of these as a measure for success may cause you to lose track of the big picture.
As an example, I hear a lot of designers complaining about the design of eBay. They point to a cluttered home page and dated aesthetics. Granted, these could be improved, but looking at these factors alone ignores the broader impact of the eBay service. eBay is the 30th largest economy in the world. 700,000 people make their full time living selling on eBay. Individual sellers can reach a global audience of millions from their home using it. If you compare those metrics against eBay's goal of creating a level playing field for sellers of all sizes, and thereby truly democratizing commerce, the service is actually a huge success of which the design is no small measure.
Unfortunately, we lack an effective way to evaluate designs of this significance. So instead, critics focus on what they know best: aesthetics. Until we develop a better way of judging interaction designs that manage things like global economies or communities, we'll continue to define success with visual design awards, which is unfortunate because only a portion of my time is spent making things pretty - most of it is spent making experiences that are useful, usable, and more desirable."

"Q: Do you consider visual design and usability to be two separate disciplines? If so, where do they meet, or how do you bridge the gap?
Luke: The visual design of a website bears the responsibility of communicating the possibilities, limitations, and state of interactions. It tells users what they are seeing, how it works, and why they should care. The better at communicating we are, the easier it is for our users to use and appreciate the websites we design.
However, the wrong message may be sent to users when visual elements are applied without an understanding of the underlying interactions they are meant to support. Visual styling that obscures or clouds crucial interaction options, barriers, or status messages can have a significantly negative impact on user experience.
You can think of visual design as the 'voice' of interaction design and information architecture, and therefore directly responsible for the usability of a website.
Now I don't think that means usability & visual design are the same discipline, but there is certainly a symbiotic relationship between the two. Bridging the gap between them requires an understanding of that relationship and its impact on the final design of a product."