Sorry, but sending some letters complaining about a cast you haven't even seen yet isn't going to get Shyamalan to recast the movie. Shyamalan's not going to read a letter and go "OH YOU'RE RIGHT! JESSE MCARTNEY IS WHITE! SILLY ME FOR NOT NOTICING" and recast the movie.
So that being said, the letter campaign is pointless, and I don't think DH should be linking to it at the top of the page. It's essentially taking a stance against the movie, and an Avatar fan forum shouldn't be doing that.
Trying not to parrot what night said, but the link to the petition was put in the header as it is something that is concerning to many Avatar (and non-Avatar) fans. It was my idea to put it in the header as opposed to just a thread, so that the information would be there for easy access for those who may be interested in it, as well as for those who are not keeping up with the movie news and might otherwise not know about it.
DH is not endorsing the petition, but that doesn't mean we can't also post news about it. I know petitions rarely work, but for those who want to do something, it may still be worth trying.