Interesting example, as MLK's estate in fact will try to make you pay licensing fees to do this, claiming they own the copyright and recognizing few instances of 'fair use'.

http://en.wikipedia.org/wiki/I_Have_a_Dream#Copyright_disput...

And attribution actually has little to do with copyright infringement (in the U.S. anyway). It is important not to confuse the legal concept of copyright from the social/academic concept of plaguirism, they are different things. Whether you have a fair use to quote MLK's speech without permission from the King estate is something you'd have to take up in court -- but whether you properly attributed it or not is not really of significant relevance in determining whether it was copyright infringement or not. (In the U.S. anyway)

(An interesting exception might be CC-BY licensed things -- which offer anyone interested an automatic license to use a copyrighted work without fee, but only if the user attributes properly. But the attribution is a term of the license, not a part of copyright law in general.)