Jacob, in his chat with MiB in "Ab Aeterno", told MiB that if MiB found a way to kill him, another candidate would simply take his place. We know that it is Jacob that is keeping him on the island, but what hasn't been considered yet (at least I don't think it has...) is that it isn't Jacob's job because he is Jacob, it's Jacob's job because of the ROLE as island protector that he has. That is to say that MiB is trapped so long as there is a protector of the island. So long as a new candidate takes the place of the previous one, MiB is trapped by on the island.
So the reason that MiB couldn't kill Jacob -- and he knew this -- is because it wouldn't free him so long as the candidates were free to take his place. So he was forced to find a way around these rules, forced to find a loophole, to find a flaw in the logic somewhere. And in the centuries he's been on the island (or at least I'm assuming it has been that long), he's come to know the rules better than anyone; and he has found his loophole in the candidates.
(SPOILERS FROM THE PACKAGE FOLLOW, BE WARNED)
Up until now we have the smoke monster take the form of a number of people we've seen on the show, but Locke is arguably the only form he has taken that has been a CANDIDATE. His loophole is precisely this fact: he now has taken the form of a candidate. Thus, he was allowed to be "invited in" to the statue and have Jacob killed.
Something Locke said tonight in "The Package" intrigued me -- when he said that he was "3 people short" of leaving the island. I propose that these 3 people he means to collect are the remaining candidates not in his party. For this is the completion of his loophole: he must get himself and the remaining candidates off the island before one of them can take Jacob's place, or else the whole system starts over again, and he must wait for another opportunity to exploit his loophole.
And a loophole it truly is, if Widmore is right. Widmore tells Jin that if MiB is allowed to escape the island that everyone and everything will simply "cease to be." This is a very powerful choice of words, and should put up a red flag. He didn't say "killed", he didn't say "perished", or even "destroyed". He said "cease to be". This implies that if MiB is permitted to accomplish his loophole to its end, that his escape off the island will somehow negate all of existence.
And that, I might add...just won't do any good.
Feel free to comment, I'd love to hear your feedback. 3rd person perspective will help me refine things.