As I was mulling over some more Lost items, I had a couple other ideas, which I'm sure will be debunked by tomorrow's episode (you never know :P ), but here they are nonetheless.
"The Rules" We're always mentioning 'the rules'. There seems to be several sets of rules, but Smokey was looking for a loophole in these rules, so he could kill Jacob. Now these particular rules, governing the behavior between MiB and Jacob, have to be some hard and fast rules and not some "gentleman's agreement". If MiB really wanted to kill Jacob so bad, he would forget any former agreement to rules and just kill him.
So the rules are enforced by somebody or something. I find it hard to believe that some physical law of nature, even on the Lost island, prevented MiB from killing Jacob (though you never know, I suppose; what's with that ash that stops him?). Perhaps "the island", which could be seen as a character in the first few seasons still is an entity unto itself. The island enforces these rules, or prevents them from being broken. Mysterious boy in the woods could well be a manifestation of the island.
Personally, I kind of doubt that. It is looking like either MiB or Jacob had a hand in most if not all strange hallucinations and such on the island. "The island" as a character was probably a combination of Jacob's and MiB's actions. But you never know; it had to be said.
Also, I've seen some comment that "maybe Jacob is the real bad guy". After all, it would be just like the Lost writers, who constantly challenge our view of Good vs. Bad, Science vs. Faith, Destiny vs. Free Will to pull a switcheroo on us like that, right? Wrong!
Locke said it right when he was explaining Backgammon to Walt. Two sides: one is light, one is dark. Jacob and MiB have been about white vs. black. And "white" doesn't necessarily mean "good", MiB and Jacob have been "light and dark" about more than just Good vs. Bad. MiB has clearly represented death; he has brought death and imitated the dead. Jacob, however, has always been about life. Apparently Jacob keeps Richard forever young, and I at least thought it was pretty clear that Jacob brought Locke back to life after he fell out of the window. (And we don't have concrete proof yet, but Jacob dies and the Healing Spring seemingly loses its healing powers.)
I don't know about you, but I want to be on the side of Life, thank you very much, so I'll keep rooting for Jacob. While we're on that subject, I think it's a decent guess that MiB represents Destiny and Jacob represents Free Will. MiB said "You're still trying to prove me wrong", to which Jacob quickly replied "You are wrong". Seems like Jacob is trying to prove something that never happens, but only has to prove once. My theory: that through choice and free will, these characters can change the course of history, change their destiny. Well, we'll see, but I'd also side with Jacob on the matter of Free Will, as well.
Can't wait for tomorrow!