I think it was when talking to Jonathan from Plagiarism Today (http://www.plagiarismtoday.com) that I was told that forum posts should be regarded as the authors' own, legal copyright property. I may have some old notes on this around that I can dig up.
Some forums (and sites like YouTube) write some Terms of Use/EULAs to waive their content rights to the site owners (not for any nefarious reasons), although I doubt that these "agreements" hold up.
If I had any comments or forum posts that were copied verbatim in a manner that upset me, I would definitely pursue it legally. But I'm a fanatic like that.
http://webcache.googleusercontent.com/search?q=cache:6GfoIFr...
I'd like to hear if anyone else agrees or disagrees with this article (it seems to be a bit of a grey area).