@Otto -

Ideally, you should put a message in the header of the PHP file itself that explains the license. This way, the license declaration travels with the file.

That's what I do, too, Otto. I also explicitly state the license (and version), and the URL to find the full text of the license, using "License" and "LicenseURI" slugs.

Simple? Easy? Very much so.

The problem, though, is that there is currently nowhere on wordpress.org/extend/plugins Developer Center that either a) explicitly states that such a declaration MUST be made, or b) gives suggestions/examples for how to make such a declaration.

There is good information in the Codex, but that information isn't even linked from the Extend/Plugins Developer Center, where the repository guidelines exist.