The HTML5 people say that the new
<aside> element is named such because
<sidebar> is presentational.
To which I present a DICTIONARY as a counter argument:
1. a short article dealing with a sidelight of a major news story and printed alongside it
2. a discussion at the bench between the lawyers and the judge, conducted outside the hearing of the jury
Coming from a newspaper design background, that is how I have always thought about sidebars.