Last nights Lost was a doozie! The quick run down is that Ben has father issues and killed off all the Dharma Intitiative peeps by gas'n them. Ben takes Locke to see "Jacob" who speaks to Locke. Locke is then shot by Ben and left for dead in the open grave of the Dharma ppl. Here's the entire run down on About.com
There are many theories floating around to the nature of the island and motivations of Ben. Some say it's hell, some say a limbo of some sort, some speculate it's an electromagnetic sensitive place. The Poprocks Blog has a good run down of theories and speculations. Another good run down can also be found on the New York Mag online.
Here's my amatuer assessment of the island's true nature. The writers have put in a lot of Stephen King references, from the book review of Carrie to numerous others.. for the full run down check out this post.. So for me to come out and say what the island is it would have to explain the following:
1. Reappearance of Dead People
2. Miraculous Healing (Locke being prime example)
3. The Black Cloud
4. How can their be crash survivors when their plane being 4 miles under the sea
In Stephen King's magnum opus The Dark Tower Series, the Dark Tower is the center of the multi-verse. The point where all dimensions and universes come together into one point. If the island is this world's Dark Tower, it would explain a lot. The island would be "thin" where long dead people can cross over and contact those on the island. Also this would explain how there could be no survivors of flight 815 and yet have survivors on the island at the same time ala Donnie Darko. This incorporates tangent universes, multiple life-locations, and even Locke's healing (because he was never paralyzed in this dimension/tagent universe).
Granted there's a lot of work that needs to go into this theory and a whole lot more research than i'm prepared to give to it. I think my buddy Jason would be the go to guy to confirm or deny, as he's the one who got me hooked on this damn show that never gives me answers!!!!