Martin Luther King's "I Have a Dream" speech is still in copyright (as is almost everything else familiar in our lives), and Dr King's heirs strictly enforce the copyright. Wendy Seltzer points out what this means for free expression and political commentary.
You can always quote a few lines without asking permission, but that's likely to be the same few lines that have become cliched with repetition. Quote the whole speech to make a more substantial point, and you face thousand-dollar license fee claims from the estate. Quote them to make a point critical of King, and you may be denied a license entirely.Link