When should a supervisor be an author?
Allow me to strongly disagree with eykanal's answer.  There is no universal standard.  You must ask your advisor in advance what her coauthorship policy is.

In theoretical computer science (and mathematics), it is generally considered unethical to list someone as a co-author who has not made a novel and significant intellectual contribution to the paper.  In particular, merely funding the research is not considered an intellectual contribution.  Adding a supervisor's name to a paper to which they have not directly, intellectually contributed is lying.

In practice, writing a good grant proposal requires at least as much intellectual novelty as writing a good paper.  Most of the good ideas that PIs pour into their proposals also appear in papers; as long as those ideas constitute novel intellectual contributions, the PI merits co-authorship.  But that only works once per idea; once an idea has been published, it's no longer novel, by definition.

To be specific and personal:


I am not a coauthor on all of my PhD students' papers.  (Of course, I still report my students' independent work back to NSF as outcomes on whatever grants supported them.  So I still get credit from NSF for having the foresight to fund the student.)  The same is true of all the other theoretical computer science faculty in my department.
My PhD advisor is a co-author on only one of the papers I published as a PhD student.
My PhD advisor doesn't have a single co-authored paper with his advisor.