Re: citing things in forums - I think that our practice is (inevitably) changing here. I have noticed that several well-known bloggers have increased the use of images in their blogs and tend to use CC Some rights reserved images (or their own) and give an informal citation to show the source.
I agree with you that we are fuzzy about self-plagiarism - I am happy with fuzzy actually. At one end of the scale, there is 'theft of an article' (probably Josep's example) and at the other end of the scale there is an accidental missing citation, and an awful lot of fuzzy in between - yet when many educators talk about plagiarism they seem to assume it is clear cut and 'beyond the pale'.
The point that I was trying to make was that I think as educators we should look at our practice, seeing how it develops as the
context changes (the privacy of the classroom giving way to the publicness of the Internet). That might help good scholarship be seen as something that student learn over time rather than the deficit approach of 'plagiarism' where I have heard some academics lump all 'plagiarism ' together and treat it as some sort of evidence of the moral decline of youth.