As I said a couple months ago, I just hope and pray that Lucas doesn't puss out here. This one has to be DARK. I mean, come on, Vader has to slaughter all the Jedi save for Yoda and Ben. Not to mention Padme has to die and shuttle the twins off to their respective hiding places.
Worst case scenario is that they END the movie with Anakin falling into the volcano (thus needing the suit). In which case, the words in the trailor from the emperor (Rise Lord Vader, RISE!) would be the end of the movie.
Best case scenario is that the fight on the volcano with Ben is the end of the FIRST ACT, which would mean plenty of time to watch Vader kick ass and hear James Earl Jones do that voice again:)
Just my opinion though.