I'm a bit late to this as I was travelling today. It's Anti-Plagiarism day for literary bloggers, and all the links can be found at How Publishing Really Works.
I'm not so sure there are many easy answers to the problem of plagiarism, since as many bloggers point out it's a matter of ethics, you can't copyright ideas, and it can be hard to prove that mimicry wasn't unconscious (the plagiarist's classic defence).
Some bloggers are keen to reassure people that it doesn't happen very often and not to deter people from taking part in web writing forums, but I have to say I feel somewhat more cynical, having had personal experience of professionals plagiarising the work of unpublished authors and new writing schemes operating as cynical ideas-gathering exercises for the media.
Still, I think that making a noise about it in this way is probably just about the best thing to do: plagiarism is made much easier when people generally aren't aware of the problem.