This theory expands on my first theory and several others that have popped up since Across the Sea aired. First, to recap my first theory (which can be read at http://theoriesonlost.blogspot.com/2010/04/mib-is-islands-protector-by-jim-cobb.html), I speculated that MIB was actually the island's protector, drawing the parallels to Cerberus, and suggesting that Jacob's task was to keep him from leaving because he was truly protecting whatever was under the "cork".
Having seen Across the Sea, it's clear that that is not entirely so, but it doesn't take too many alterations to get back on the right track. It's been suggested in another recent theory that MIB/Smokey has taken the light/source with him from the waterfall. This is in line with what John Locke said I believe in Season One, that he saw a "beautiful light" during his first encounter with Smokey. When MIB said to "Mother" that he was going to channel the water and the light, it resonated with me and I've been thinking about that line since. When I saw the new mystery posted on DarkUFO, "Why was the Donkey Wheel not frozen…" (http://lostmysteries.blogspot.com/2010/05/why-was-frozen-donkey-wheel-place-not.html), it seemed obvious to me. What happens to water without a warm, bright light? It freezes.
Our Island/Universe has been in trouble and unstable since Jacob threw MIB into the waterfall. And it seems to me that the only way to right this is to send MIB back into the waterfall to join the light with the water again. How will anyone convince such an angry, deeply hateful (and possibly rightly so) entity to make the ultimate sacrifice? Perhaps that is the true purpose of the Candidate. To show MIB the light he's got buried deep inside his anger and resentment. The integration of duality into oneness, which in many spiritual traditions is the definition of enlightenment. To move from a state of twoness to oneness. At this point, I'm kind of rambling away, but I felt it important to expand upon my original theory because, as Jeffery Lebowski once put it, "New s*** has come to light".