I'm now curious how many of these intersect with the crowd that thinks when I've released my own code under the GPL, that I must never make use of that code (that I own the rights to) in some proprietary project.

  > Not having a LICENSE.txt does not make Github projects 
  > public domain. 

  > Do I have to register with your office to be protected?
  >
  > No. In general, registration is voluntary. Copyright 
  > exists from the moment the work is created. You will 
  > have to register, however, if you wish to bring a 
  > lawsuit for infringement of a U.S. work... [1]

This is why open source projects often require a "contributor agreement" when submitting patches.

[1] http://www.copyright.gov/help/faq/faq-general.html

Often "contributor agreements" are only desired by (large) projects so that the large project owns the copyright on your contributions. This will allow them to change the licence at a later date (if they want). Without copyright assignment to them, they have a much harder time changing licence.

There are some cases where relicencing is good (OpenStreetMap had to change it's licence, and had to delete old contributions), and sometimes when it's bad.

http://en.wikipedia.org/wiki/License-free_software

http://ar.to/2010/12/licensing-and-unlicensing

(Note: GitHub ToS says if you put copyright code on it, then you are by default allowing others to fork & download it, so the default licence is looser than the legal default)

From https://help.github.com/articles/github-terms-of-service:

  > ...by setting your pages to be viewed publicly, you agree to allow
  > others to view your Content. By setting your repositories to be
  > viewed publicly, you agree to allow others to view and fork your
  > repositories.