I just finished watching the first hour of the season four finale of Lost. As is customary for me, I immediately open up the Lostpedia Theories for the episode. I came across this little bombshell:
The Black Rock was packed with explosives. The Kahana is now packed with explosives. These situations mirror each other. The Black Rock was actually looking for the island long ago and a saboteur planted explosives. This little Spy vs. Spy "game" between Ben and Charles Widmore is being played out over centuries.
I think the Black Rock has a lot of significance. I can't wait to find out what it is!
[UPDATE: Another hilarious theory: "Moving the island is not a physical thing, but an emotional thing. Locke is clearly going to read sweet poetry to the island and the island will be moved."]