My wife and I were wondering the same thing, where the rest of the cast from LOTR was. I can only assume they were filming a movie somewhere or something along those lines, because you would sure think more of them would have been there otherwise.
Back on topic now, I am really glad that Return of the King won what it did. I really went into the night hoping that it would win best picture, and Jackson would win best director. To go 11 for 11 is just outstanding, and very deserved I think. We finally got a chance to sit down and watch the EE of FOTR (TTT is next weekend hopefully), and it just amazes me how great each of these movies is. There isn't a stinker in the bunch, and I think that is a true credit to Tolkien, Peter Jackson, and the entire cast and crew. It isn't a movie trilogy where I would say "nah, let's watch one of the other ones", because they are all very well done, in my opinion. Anyways, very happy for all those involved, they deserve all the credit in the world.
Also...I was hoping that Depp or Murray would have gotten best actor, but I guess I wasn't surprised they didn't.