See I'm not at all convinced the island was blown up. In the Premiere it showed the island as being under water in the ocean, but all the structures such as the dharma houses, the statue leg etc were perfectly intact. If a hydrogen bomb was detonated on the island it certainly wouldn't sink it with all the structures perfectly intact as much as it would blow it to kingdom come and you'd be left with a large crater at best.

That said, I'm not sold on the idea of it being the future either but I guess its feasible, LOST is pretty notorious for crazy plots and some sort of mass memory wipage by the island and sinking itself by whatever mechanism is certainly no more crazy then much of what has gone on so far.