The most wide spread belief (with some credibility lended to it via several tangential comments by The Great Flanneled One himself) is that Anakin and Obi Wan get into a light saber duel in a volcanic pit or cave. Obi Wan bests Anakin and either won't/can't finish him off or believes Anakin is dead.

Because this duel has been orchestrated by ol' Palpy (AKA Darth Sideous AKA The Emporor) through manipulation of Anakin (ATOC: Palpy to Anankin "You don't need their guidance, I can see you already becoming the most powerful Jedi ever."), Palpatine is there to rescue Anakin.

James Earl Jones has already confirmed that Lucas told him his voice will be used in EPIII, albeit rumor says only for the last few minutes of the movie.

I'm with Delbert on the background stuff to be CGId in front of. We see up close pics and vids of violent eruptions all the time. No biggie to get that stuff, it's just important to get it while you can. For safer REALLY up close lava shots, they COULD go to Kiluea, but you wouldn't get the lava exploding everywhere.

MrPex