As one who deals with very tricky matters of copyright law every single day at work, I can assure you that Farenheit 451 and Farenheit 9/11 are incredibly similar, and that Bradbury would almost certainly win his case if he ever made one. Trademarks and copyrights are defined very loosely: you don't have to be exact in order to infringe on a copyright, just "confusingly similar". I, like EUOL and Morag, immediately assumed that Moore's movie was Bradbury's movie when I first saw the name; beyond that, pretty much ever well-read person in the English speaking world made the mental connection when they saw the title, even if they didn't actually confuse the two. That mental connection is exactly what Moore was looking for, obviously, but it's a strong enough connection to get him in legal trouble should Bradbury decide to sue.
That said, he could conceivably defend his title with the parody clause, but if he labels his film a parody I don't know what that says about its nature as a documentary.