Peter explained that the actual book The Two Towers is too short for a film so they have expanded a lot of the material. He emphasized that the film will be heavy in the development of Frodo and Sam with Smeagol and that there is a lot of development with Frodo and Faramir. So much in fact, that Shelob has BEEN MOVED TO RETURN OF THE KING. I can understand that. If you remember, the last 1/3 of RoTK is the Epilogue about what happened to Aragorn and Arwen and some other characters. Nice info, not necessary for a movie.
I don't see any problem about Shelob being moved to ROTK; the story was cut off at the end of TTT and begun again in RoTK. It only makes cinematic sense to keep it all in one movie, kind of like they did with the 'burial' of Boromir putting it at the end of FoTR rahter than at the beginning of TTT.
I know that it might not make sence now, but when we see the finished product in 6 or 7 months time, I'm sure we'll all agree that whatever Peter is doing, it's a good thing.