If you want to be really sure a piece of software is GPL, it would probably be alright to include a file in your source tree on your harddrive saying that whoever finds it is free to make a copy of the project for themselves and consider that you have distributed it to them. It's about the clear intent to distribute.
That's a logical way of looking at it, but it disagrees with the GPL FAQ. Note especially "If the version has been released elsewhere, then the thief probably does have the right to make copies and redistribute them under the GPL".
Plus, you can have a unilateral contract that's open to anyone willing to accept the terms. You don't have to take any action per person to 'establish' it.
However I was wrong about contracts vs. licenses. Obviously I am not a lawyer.
http://www.groklaw.net/articlebasic.php?story=20031214210634...