Only a sentence in very rough terms as he's just stringing together an arbitrarily large number of noun phrases. The palindromes are of the form:<p>S -> "A man", "a plan", (NP,)* "a canal, Panama!"<p>which I suppose you could call a sentence.
Not loading for me.<p>You guys know the Weird Al song, right?<p><a href="http://www.youtube.com/watch?v=Nej4xJe4Tdg" rel="nofollow">http://www.youtube.com/watch?v=Nej4xJe4Tdg</a>
Google cache: <a href="http://webcache.googleusercontent.com/search?q=cache:7e4IeWgkyioJ:norvig.com/palindrome.html+http://norvig.com/palindrome.html&cd=1&hl=en&ct=clnk&client=firefox-a" rel="nofollow">http://webcache.googleusercontent.com/search?q=cache:7e4IeWg...</a>
That's AFAIK the longest human-made palindrome but I'm sure there must be longer ones:<p><a href="http://everything2.com/index.pl?node_id=830538" rel="nofollow">http://everything2.com/index.pl?node_id=830538</a><p>I'd be glad to find longer ones!