This is alluded to in other posts, but most directly here, so will answer here:
> I wonder about the original post in this thread, because at some point in the discussion it has been explained that the CC-Organisation did tell us exactly this when asked and that we should use CC0.
This would have been someone from 'Science Commons', a brand of Creative Commons intended to speak to ... science, where it was deemed a more normative approach (public domain only!) was needed. That stance wasn't intended to apply to all fields, though lack of statements from CC on use of CC licenses for data doubtless helped create the impression that public domain only without nuance was CC's universal recommendation.
In the meantime, lots of projects were using CC licenses for data (including OSM, and some really old ones such as MusicBrainz, which has split its data between what it considers purely factual and in the public domain, and user contributed annotations, which are under BY-NC -- for better or worse -- approximately from the very beginning of CC) and CC did not pay adequate attention, at least publicly, to those users. I apologize for that.
We are just starting to rectify that with regard to data -- or I probably wouldn't be posting in this thread; also watch the series starting with http://creativecommons.org/weblog/entry/26016 -- and we've also learned some things about avoiding messaging confusion. Over the course of 2010 we retired both our education (ccLearn) and science (Science Commons) brands because the Creative Commons brand is more powerful and speaking with a unified voice forces us to be more rigorous and less confusing -- please spank us if we fail at this. :)