The Mommy Blawger asked me something that I'd wondered about, too: Why do legal works tend to use Id. to mean "same source as last time" rather than the Ibid. that's more common in other disciplines? I asked my brother Sasha, who is generally The Man Who Knew Too Much but in particular my Latin expert. Here's his answer:

"Id." is short for "idem," meaning "the same (man or thing)." "Ibid." is short for "ibidem," meaning "in the same (place)."

So you can find usages where "id." is for people and "ibid." is for books. For instance, see here. So I've seen things like "Jefferson, Book A" and then "id., Book B."

While that last source says "ibid." should stand in for the entire source (author + book), hereyou can see someone using "Id., ibid." to mean "The same author, in the same book." here you can see "Kant, ibid." where "id." would be inappropriate for Kant since the preceding source wasn't Kant. That's problematic (just like another old-style citation term, "op. cit.", since the reader might have to leaf through several pages to find the last time you cited Kant and see what book you were talking about),

Some (for instance, here) say that "ibid." means "EXACTLY in the same place," so you can say "ibid." for the same page of the same book, while if you're changing pages you should say "id. at 581" or similar (see here).

The bottom line, though, is that even though some disciplines use both "id." and "ibid.", and various style guides will explain the difference, there's really no reason to have both of them. Of course, the Bluebook has entirely dropped the distinction; and I read a president's introduction to an old volume of Law Reviews where he announced that they were finally dropping the "id."/"ibid." distinction which no one could really understand anymore.