Lawyers-R-Us?
Sure there are some general rules of thumb, but the obvious answer is that if the content is copyrighted you are not "legally" allowed to use the content in any sampled format whatsoever without written permission. Now the question of whether or not anyone will ever even know you exist and that you sampled something, let's be real...
Try Google:
http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=legal+sampling
http://www.outersound.com/osu/publishing/sampling.html
As far as the public domain use of public discussions, etc. I recently read some legal articles around the use of video footage that had been taken in public environs, e.g. Mardi Gras, Spring Break, Burning Man.. get the picture? Well, Burning Man is private property and they explicitly state that you cannot use vidoe footage or pictures for commercial purposes, but the rulings I read regarding "Girls Gone Wild" videos was that if your it in a public space, the video footage becomes public property and can be used commercially (at least in the Mardi Gras case, now this might not apply to "all" public spaces, but this particular space the judge said participants knew they would be filmed).
The worst that could happen is you'd get a cease and desist, and if you become that popular, perhaps the negative attention would be a good thing, just hope you don't sell a million records with a sample that gets caught, that's a lot of lost revenue. ;-)
Search the legal sites on the web if you sincerely want legal advice.
