Is there a citation in this text?

IHE recently linked to posts at Language Log and Savage Minds on the question of "citation plagiarism"--that is, lifting a quotation from text X out of secondary source Y without citing Y in your own project Z.  The issue arises, of course, in the context of the Finkelstein-Dershowitz debacle. Bill Poser gives the entire idea of citation plagiarism a thumbs down, while Kerim Friedman believes that we jettison the concept at our scholarly peril. 

Poser reminds us that "we cannot tell from such a citation is whether other authors have cited the same work and how the author came across it. So what?"  But there are three different issues here:

1) Using a quotation without providing a "qtd. in";

2) Using an altered quotation (whether by deliberate omission, addition, silent combination, or the like) without providing a "qtd. in."

3) Using an altered quotation (because of some mistake down the line) without providing a "qtd. in."

Case #1 is, as Poser says, untraceable.  But unless Scholar B checks Scholar A's quotation against the original, she cannot know if she is actually engaging in case #2 or case #3.  (I confess to being a little skeptical about how often authors double-check their "qtd. in" citations.)  And I think that case #2 does, in fact, constitute plagiarism: once altered, the quotation in effect "belongs" to two different authors. In any event, cases #2 and #3 raise all sorts of questions about context, authority, and so forth--what if the quotation's meaning is altered with the addition of the next sentence?

More to the point: aren't most of us trained to reserve "qtd. in" for emergent occasions, to deal with texts that we have no chance of seeing? Medieval manuscripts in Icelandic monasteries are one thing; Michel Foucault is quite another.