I really find it hard to believe that people think Anakin will fall into and completely get submerged in lava.
I also want to shoot those volcano planet theories down once and for all...
First some facts about geology: at the cores of most planets (definitely earth) is molten lava. Coruscant is likely the same way. In mythology, the hero's journey through hell takes him in the direction down. In religion, hell is also referred to in the direction down. Coruscant is a planet full of vice and corruption and to sink as deep as one can get in it would set one on a course for hell. Palpatine will most likely preside over part of Obi-Wan and Anakin's duel, and to escape assasination and for private meditation away from where the Jedi can sense his powers, it would make sense if he had a secret lair beneath the surface of the city-planet. Somewhere even the granite slugs and sub-human demon lifeforms that roam the lowest, most uninhabitable levels of Coruscant don't dare go because of either a lack of access, or the fire and the heat. Besides, it sets it up perfectly that these creatures exist in the darkness just above the entrance to hell - demons are always said to protect it. And I'll add - we never see Palpatine leave Coruscant.
Next some facts about Darth Vader (that we know for sure). Vader has a mechanical right arm. Dooku cut his original biological arm off. Vader requires a respirator unit. This kind of thing helps people with lung damage. You can see less portable versions in hospitals today, as well as less complicated portable ones with the elderly who must carry oxygen with them. Vader has some lightsaber scarring to the back of his head and face - Obi-Wan Kenobi has a lightsaber (duh). Vader has the powers of telekenisis - the ability to move things with the Force.
A fact about molten lava: it burns up and disintegrates everything. Watch Terminator 2 again and see what happens when the T-1000 falls into molten metal (lava is the same thing - the earth is composed of super-heated molten metals - that's what lava is). Flesh would start to burn even before you get into the lava. (See Indiana Jones and the Temple of Doom) - oh and Anakin's flesh is not burned! (see Freddy Kruger in A Nightmare on Elmstreet). Fact: the air above lava is super hot. Fact- people, human Jedi, breath air. If Anakin breathes this super-hot air into his lungs, wouldn't he damage them? Fact: even if Anakin was too close to molten lava, he'd still have to breathe. Fact: Obi-Wan does not get that close, obviously.
Finally, theory: if you had even unprecedented powers of telekenisis, they would be hard to control after you got struck in the head by a lightsaber - and twice at that! We've never seen a Jedi project a "Force-field" around them, using their abilities of telekenisis, to shield themselves in a 360 degree direction. Anakin would not survive being submerged into lava if he did not attempt to do this. He would be burnt up anywhere that he failed. It would only take seconds and he could not be saved. Therefore, he will not fall INTO lava. But it may appear to look that way.
Lava comes up from cracks in the earth's surface. Cracks look like cliffs - the ocean's chasms were formed that way. There are ledges and precipes on the edges of cliffs that can't always be seen.
I submit Anakin falls over the edge (without his new lightsaber), and to Obi-Wan it looks like he went in (like when Jango thought Obi-Wan himself fell into the drink on Kamino during the rain fight). Anakin did not fall in - he landed on a precipiece. He may see that much, then the flames rise so it obscures our view of Anakin on the ledge. We know he breathes the air down there - too close for his own safety. But that is the last we see of them.
FACT: this is a 12-13 hour movie with chapters 1-6. If we see Anakin being rescued and put into the Vader armor, it will ruin Episode 5 when Vader reveals to Luke that he is his father.
If watching it for the first time, we don't even know that the new hero is Luke SKYWALKER, because he doesn't even say his name until he rescues the Princess on the Death Star.
He tells Threepio "You can call me Luke. No, just Luke."
In fact, we don't even know that anyone on Tatooine DOESN'T think his name isn't Luke Lars up unto that point.
Vader will appear in Episode 3 after Anakin is presumed dead by the characters in the movie. We will "know" he is actually Darth Vader, and the Jedi characters will sense something familiar about him, but we will not be shown Hayden Christensen in the black suit (even if the actor wears it to play Vader in some parts).