Section 102 of the Copyright Act of 1976 provides that copyright applies to "original works of authorship fixed in any tangible medium of expression, now known or later developed, from which they can be percived, reproduced or otherwise communicated, either directly or with the aid of a machine or device." Something is "fixed" in the tangible medium of expression when it is "sufficiently permanent or stable to permit it to be perceived, reproduced or otherwise communicated for a period of more than transitory duration."
This would seem to indicate that Internet postings are within the copyright laws. Keep in mind that something fitting the definition above is eligible for copyright the moment that it is fixed. You need not apply for copyright protection (although you need federal registration to enforce your copyright).
However, also keep in mind that "ideas" are not copyright-able. Copyright is limited to the author's "method" of expressing the idea. Thus, a Seinfeld writer could get an idea for a script from an Internet posting without infringing any copyright. However, taking a humorous line directly from a posting and using it in the show could be seen as infringement.
There are all kinds of problematic intellectual property issues involving the Internet. So far, the medium is too young to have been fully addressed by the courts or legislatures. At this time, they can't even figure out when a computer program has been infringed.