...when Alex died MIB was freed... by FJA243

The basis of this theory is trying figure out when the loophole that MIB took advantage was actually opened. I know his plan was in place long before any of the events that I'm going to talk about happen. I believe that Ben's words 'he changed the rules' is biggest clue that this the moment when things are 'changed' enough for MIB to take advantage. I believe that just before or just after Alex died is when MiB is enabled or freed to take over Locke's body on his return in Ajira flight 316.

I know that the gap in the ash surrounding the cabin which is found by the gang carrying Locke's body is a clue that the cabin prison is no longer holding MIB inside. Just removing the ash in one area of the circle of ash seems too simple...but perhaps in season 6 we will learn that MiB as Christian tells Locke to remove the ash that simply. Otherwise, Hurley, Ben, or Locke while looking for the cabin just trip and kick the ash away by mistake but I believe that is unlikely.

I realize that Ben may be referring to Widmore but that is unlikely because the rules aren't set by Widmore...from Ben's perspective the rules are set by the man he considers in charge...Jacob. Widmore and Ben just follow the rules. This the seed that is planted in Ben which begins to erode his loyalty towards Jacob. Prior to Alex's death Ben is pretty calm for guy being hunted down by a bunch of mercenaries and he communicates with the 'others' with the mirror perhaps setting up the ambush of Keamy and co. However, the moment Alex is actually killed Ben's cold-calm-rational demeanor abruptly stops and he is genuinely shocked that this happens...he is even willing to 'teleport' from the island to get revenge on Widmore. More importantly, he summons the smoke-monster within the Dharma compound to hunt Keamy and Co. Up to that point the smoke monster couldn't infiltrate the Dharma compound because of the sound-barrier fence. The smoke monster is summoned by Ben for his own ven! geance, not to 'protect the island' or for Jacob. And this summons of the smoke monster, I believe is special or different from other instances which somehow frees the MIB to be able to complete his plan of sending Locke out to fail to get the Losties, die, and be brought back to have his body taken over by him...and manipulating Ben.