Take this passage from Abide the Sleeper, the novel imadra_blue and I recently co-wrote:
The green aura around the locket pulsed and glowed as a living, breathing thing. The violet light stopped flowing. There was a high-pitched humming noise—whether in Draco's mind or in reality, he wasn't sure—and the glow around the locket exploded into a million points of sharp light. Not a moment too soon, because Draco lost control of his wand and it dropped to the grass at his feet. He sank to his knees and hid his face in his hands, not caring what the others would say. None of them were meant to be doing this, it wasn't right.
Compare it to:
The locket's green aura glowed and pulsed like a thing that lived and breathed, and then the violet light stopped flowing. A high-pitched humming noise sounded in Draco's mind -- or was it real? -- and the locket's aura shattered, a million sharp points of light piercing the gloom. This couldn't have happened at a better time: Draco finally dropped his wand, which fell to the grass near his feet. He knelt beside it and covered his face with his hands. It didn't matter what the others would say to this display of weakness. This was not right. They were not meant to be doing this.
These passages tell exactly the same story in different words, and you can actually find the original by Googling bits of the altered version (using normal search, not phrase search). But what if the details were changed? Make the locket a ring, the green aura gold, the violet light acid-green, the humming noise piercing instead of high-pitched, etc, and then spend a few minutes with a thesaurus to change recognisable key words -- and you have successfully stolen what someone else (me, in this case; I wrote this particular passage) imagined. Only someone who had read Abide the Sleeper -- and read it carefully -- would be able to tell that the second passage is a rewrite.
Of course, you get arguments for 'postmodernist pastiche' (which I understand it as the notion that every idea in every genre is 'recycled' through time and thus borrowing from prior sources becomes unavoidable -- please correct me if I'm wrong), but I think there is a considerable difference between exploring ideas through writing and reusing someone else's exploration of these ideas by rewriting. Since the number of ideas is finite (a very large number, but finite, as long as we are bound by a) the physical constraints of our universe and b) the limitations of language), it stands to reason that somebody, somewhere, has had the same idea as you, and perhaps has even written something about it. It might be on their obscure blog somewhere in cyberspace, or it might be sitting in their desk drawer among other similar writings.
Synchronicity in thinking is not new; people have similar ideas all the time -- and sometimes, especially in fandom, the same ideas get ridden to death by multiple authors. There are only so many ways to tell a specific story, and I think in the case of your average hurt/comfort Harry/Draco scenario, all of these ways have already been explored in one fashion or another. Obviously, people will come up with fresh storylines and plots to support these scenarios, and they will use different words, different shades of characterisation to bring their own ideas to life, but this is what I think is essential behind the whole notion that ideas cannot be copyrighted. Words can be copyrighted, of course, but where do you draw the line between "idea" and "words"? The passage cited above contains several separate ideas, all tied up in some fashion with the rest of the story (and with the forthcoming sequels), but it can be easily used as a jumping-off point for a completely different story that ties these ideas up in a different way. I could write a story around it that has nothing to do with Abide the Sleeper, and if I can do it, anyone can. Is that still plagiarism?
The passage I cited above -- I sat with my laptop in much_reality's guest room last December and just wrote what I pictured in my head. That I don't know of anyone who's written anything similar doesn't mean that no one has, does it? If someone has, I can choose to acknowledge it or not, because I didn't know of it at the time that I wrote the passage. Plagiarism happens when you see something someone else has written and like it so much that you decide to essentially borrow their thought process, or reuse their words -- but how do you argue against someone who says "well, s/he said it better, and I saw no point in trying to reinvent the wheel"? My bog-standard response would be that if you seriously think someone else has already done something better than you (so much better that you don't feel you can do better), you shouldn't attempt to do it yourself and just be content that it exists. But that's me. What do you think?
Do you think it's plagiarism to see a particularly sharp turn of phrase in someone else's work and begin using it in your own, because you liked it so much? If you look at erotica, you'll see things like "fingers ghosted over [something]" and "bruising kisses" and "captured [his/her] mouth in a kiss" -- aside from being horribly cliched, these phrases had to have come from somewhere. And if something has become a cliche -- and you're the type of writer who defaults to cliched phrasing -- is it plagiarism to use it in your stories, if you borrow a phrase that's standard to a number of writers rather than one specific source?
My "final" stance on plagiarism is basically "thou shalt not steal". If I see a phrase that makes me go "damn, I wish I'd thought of that!", I will turn green temporarily, grumble a little, and then actively avoid using that phrase in my stories. Sometimes I feel like there's a catalogue of these in my head; the one that comes to mind right now is pir8fancier's "those little brown crinkly paper liners used to protect chocolates from one another" (from WitC6). I might not always remember to whom a phrase belongs, but I will very likely remember that it isn't mine. Sometimes I probably slip up, because memory is not infallible, but my philosophy is that the best way to avoid being accused of plagiarism is not to plagiarise -- that is, knowingly and wilfully using someone else's words and passing them off as your own. Even if you spend more time trying to make them unrecognisable than the author spent writing them. On the other hand, I think if you admit to borrowing this or that phrase upfront -- that is, in your header, or in footnotes -- that's not plagiarism. But I would draw the line at borrowing turns of phrase; I can't fathom why anyone would borrow whole passages.