[Dénes Bernád]

I would differentiate between original photos (prints) and original negatives.

Obviously, all existing prints (and their reproductions) of a certain subject can be traced back to the original negative. Therefore, if someone owns the original negative - even if he did not actually push the shutter release button - can fairly claim that he holds the 'source' of prints made off it.

Now, I am not certain that owning the original negative is equal to owning the copyright, but chances are much better than simply owning a print.
I'd say - without any legal backing, however - that if someone owns the original negative can pretty much claim the copyright over someone who owns a photo made from that particular negative.

Obviously this does not apply to the current era of digital photos...

P.S. Very interesting thread, BTW. I agree with Ruy, that the topic should be made 'sticky' and periodically updated, when new and reliable information comes to light.