I think PJ knows what he's doing. The movie is pretty long, and I bet the Pelennor Fields will be somewhere about halfway into it. It will take at least half an hour, and as soon as its over and everyone is winding down from watching the hugest battle onscreen ever, they'll start preparing for the battle at morannon. While they are about to fight it though, the focus will definitely be put on frodo, sam, and gollum. And don't forget, PJ said that his climax at mt. doom is different from the book. He said Sauron will be more than just an eye, but something better than just a big guy in a big black costume.
I also think that PJ will do something to show how united Middle Earth can be. he put the elves in TTT, so he might put elves, dwarves, and possibly Ents in one of the two major battles, if not, then both.
It would be cool to see all them fight together side by side against the "orcses". Keep in mind that the books, the elves and dwarves were not hiding. Lothlorien was under constant attack, but never invaded cuz they rock. The dwarves and elves of mirkwood (legolas' buddies) were attacked by an army almost as big as that in the south against minas tirith. And, unfortunately, the elves of mirkwood with the dwarves lost their battle. it was only after the ring was destroyed they were able to repel the orcs.