The real reason why Shelob isn't in TTT is because...

I figure the real reason Shelob isn't in this one simply because frankly, they need something to show in the third movie.

I doubt peter jackson has any fear that a similiar creature might be in harry potter. If that was the case, I don't think we would see much Wizard related folks in the movie.

Shelob is referenced by Gollum, albeit in a way that only readers of the book would know what he was talking about.

Also it would be a major cliffhanger considering at what point TTT the book ends. He didn't want to do that for Fellowship which is why we find out what happens to Boromir in FOTR. Beside, Shelob would be a big antagonist for Frodo and Sam to begin ROTK.

I hate spiders. Shelob sounds like a really disgusting creature in the book. I can't say I'm looking forward to seeing her. Yucky.

Don't forget, even though there are three books, almost half of the third is the appendix. They probably needed a little bit more story for the third picture.

I don't know. Think how much screen time will be eaten up by the battle of the pallenor fields. If you consider just how much grander in scale it is than the battle at helms deep (which consumed a good 45 mins of screen time) I have a feeling RoTK will turn out to be the longest of the three films, especially with the additions of the chapters left out in TTT (Shelob's lair, The Voice of Saruman, The Palantir etc)

It was primarily to give Sam and Frodo something to do in the 3rd film. Without Shelob it would be just more of them wandering to the Volcano. Especially with the Scouring gone.

What? didn't Jackson throw a scene in where they come across and 'liberate' some nekkid elf chicks?

Going by the "old" paperback edition page numbering, there are about 360 pages of story in ROTK. Of that, only about 70 pages are devoted to the remaining journey of Frodo and Sam. This brings a couple of interesting things to my mind.

First, as other have said, if the Shelob segment had been included in TTT, much of the remaining Mordor story would be mostly uneventful -- which is not meant to imply that it is uninteresting in the book version, because it is very interesting, just not something that will take a lot of screen time to represent. Therefore, something is needed to add some suspense and tension to that story line. Shelob seems to be the perfect addition, and I really don't have a problem with the way Jackson has handled it.

However, there is another point that no one has discussed yet, that is much more interesting to me. In the books, the quest ends with still more than 100 pages of story yet to unfold. Even with the Scouring segment excised, that is a lot of "anti-climactic" story to work through, and it could represent as much as 40-45 minutes of screen time. Do you think Jackson will preserve the post-quest storyline, or will more "liberties" need to be taken to make the events more cinematic?

The Scouring of the Shire will not be part of the third film, but I don't know about the other post-climatic events.

Tony Shaloub is not in TTT because he was unavailable at the time due to Monk shootings. Peter Jackson wanted to start cutting the film so they just decided to push him back to ROTK.