Well, I've never been on a LOST forum before and I've never heard anyone else's theories. I'm not nearly as informed as the rest of you, so don't make fun of me too badly!
I believe the island and however many miles of ocean surrounding it is some sort of nexus holding together all times and dimensions.
And it seems to me that after the hatch imploded that the very fabric of space and time may be unraveling.
I base a lot of this theory on things like the "Hurley Bird" which I believe is an archeopteryx (a prehistoric bird) or the four toed statue, which can be any number of things, but none of them seem to fit into "our" dimension.
Personally, I think it's either something created by people in a post-apocalyptic future or by beings from an alternate dimension.
Or how Jacob (who I don't believe is the smoke monster) is wearing colonial style clothes. Is he a ghost or an imprint on space/time itself? I don't know, but he's obviosly operating outside of conventional time and space.
Or even the bones of the polar bear with the Dharma collar. As I recall, they said that the bones were hundreds of years old.
Now, how does something like that occur? Could be the island finally dissipated somewhere and sometime ... or even everywhere and every time ...
Those are a few examples out of many, many others that led me to this way of thinking.