I was talking purely from personal preference, but would be interested to know if there were any studies done on this.

Widescreen movies on a 4:3 aspect TV always use black edges, but maybe that's just tradition or to do with contrast or something like that.

EDIT: Just noticed that HN has a grey content area with white around the edges, which I like, so maybe my preference is more to do with having different colour sides than dark sides - i.e. some kind of boundary to the content area.