Jakob Nielsen is a renowned usability expert who has written several books on the subject.  In his most recent post Hank Williams calls that expertise into question due to the unattractiveness of Mr. Nielsen's website

Unfortunately, I have to say, Jakob has perhaps the worst site design I have ever seen. It is as if, while he is handing out the Oscars, he is wearing a plaid polyester suit. In truth his site is fine from an information architecture perspective. But from an aesthetics perspective it is awful. And aesthetics is important in UI. If you begin to look at something and want to avert your eyes, the site has failed.
In truth, Jakob's site has always been ugly. But this time it felt, to me, beyond ugly. I looked at the bulleted list of the top sites, and the bold cramped disorganized looking type starting each bullet, and I could not bear it.

...

I am sure some, and perhaps many of you will not think it is as bad as I think it is, but I defy any of you to say it is good. And so I ask the question. Do you judge a hair stylist or barber by his or her hair? Do you judge a cobbler poorly if he (or his kids - thanks Jeneane Sessum) has old beat up shoes, or none at all? (Ok so we don't have cobbler's any more but the cliche still works.) Do we judge Dr. Phil poorly if he gets a divorce?
And do we think less of Jakob Nielsen as an interface consultant if he seems to have no taste?

Two things here...

First, Usability experts aren't Graphics Designers and anyone in software should know that the two are different beasts.  So while I think Mr. Nielsen's site could look a little better I don't think it calls his expertise into question when some people find it ugly.

Some people being the operative phrase there. 

The big thing that Mr. Williams misses and why aesthetics really aren't part of UI design is because appearance is largely personal preference.  One man's gawdy can easily be another man's beautiful.  As one example take Mr Nielsen's own site.  People generally don't put up sites that they think are ugly so I have to assume Mr. Nielsen likes the appearance of his site as much as Mr. Williams hates it.  Because aesthetics are all in the eye of the beholder.

Which brings me to my second point...In Software Design Usability should have as little to do with appearance as possible.

We are in the middle of a transition in software design from a strategy that highlights appearance to one that highlights functionality.  Technology like CSS and ASP.NET's Themes allow the developer to empower the user to customize an application's appearance which is good for everyone involved.

Again, one man's ugly is another's beautiful.

In this sense Mr. Nielsen's site proves to be an even more effective teaching tool for developers in that it eschews fancy icons and pretty fonts in order to demonstrate basic usability principles.  Given developers are Mr. Nielsen's primary audience I'd say that makes his site's appearance a plus more than a minus.

I'm all for developers working on having an attractive default appearance for their application.  It never hurts to make a good first impression and statistics show that many users will stay with the default appearance long term.  But it's important that they remember graphic design work is separate from designing the program functionality itself.  That way they'll have applications that can give the user power over his or her visual environment and in doing so appeal to even users whose visual tastes are dramatically different from their own.