A few points ...
The main one:
1) Once something is written (paper or electronic), it is protected by US copyright law. Period. The intellectual property rights are owned by the poster (regardless of their "name"). You must get permission to use that person's quote, however, I agree that in an internet environment populated by "fake names", the likelyhood of being sued is extremely low. It most likely would not hold up in court.
OK, a few other notes ... where things get fuzzy.
2) Number 1 above is the key, however, I've seen the Legal Disclaimers on various boards (EZBOARD, for example) state that in order to use any content from any forum on EZBOARD, you need EZBOARD's permission as well. Again, the intellectual property rights are owned by the author of the quote. If EZBOARD gives you permission to reporduce information contained in its posting environment, YOU STILL would need to get the originators authorization.
3) Most publisher's will recognize this law when reviewing your manuscript. Even the POD's. Keep this in mind when you submit. You may be asked to get the appropriate forms before they will publish.
4) There is a "loop hole" in this whole copyright law stuff. It's called "Fair Use". I don't want to even get into this as there are so many ways to spin the legal definition. But, feel free to do some research in this arena. Let me help point you in the right direction ...
www.prairielaw.com/messag...21&mbId=51
The above is a great resource for getting free legal advice on intellectual property. I think you will find the answers to many of your questions. I hope this has helped some and has not scared you too much.
In all, I understand that this might be a very difficult task in an environment populated by faceless names and alias'. But, the law is the law. Again, the risk of getting sued over the unauthorized use of a quote obtained on an internet message board is low, but, taking that risk is up to you. Good luck.