To the best of my knowledge it has not been confirmed by any official source outside of the show.
Neither has it been "confirmed" within the show, although it has been hinted at.
There are a couple of ways that the MIB may or may not have assumed Christian's form.
1. The "suit-wearing" Christian: Jack sees this version of his father both on AND off of the island. Hurley also sees him once in the rocking chair of Jacob's cabin. There may or may not be a difference between this Christian and the "casual clothes" Christian seen since Claire was taken in Season 4. This is the "good Christian / bad Christian" theory.
If you assume that the two Christians are separate, then the "suit-wearing" Christian would probably NOT be the MIB in Christian form, due partially to the fact that this version is seen off-island by Jack (and it is hinted that the MIB can't leave the island).
2. The "casual clothes" Christian: Claire is taken by this form of her father and it is also the form that appears to Michael on the ship, Locke during his various interactions in Season 5, and Sun at the Dharama Barracks. It has been heavily hinted that this is the MIB.
There are reasons to doubt this though, namely his appearance to Michael would imply that the MIB both has the power to materialize off of the island (although he materializes on Hydra island as Locke and the ship was within the island's "bubble") and that he has the power to "protect" people off-island...since Michael could not kill himself because "the island wasn't done with him". Also he seems to help Sun by showing her that Jin is in 1977. It could be though that his instructions to "wait for John Locke" were meant to push her into following the MIB/Locke in the same manner that MIB/Alex did with Ben.
3. Christian's missing body: Another theory is that Christian's body was "infected" in the same manner as Sayids and that this is the "casual clothes" Christian and/or the "suit-wearing Christian" we've seen all along. One problem with this idea though is that unless the dead-infected people have the power to materialize at will and time travel (as Christian did both to Michael on the ship and Locke at the frozen donkey wheel) it is unlikely that Christian's actual body is up and moving around. Even if this were correct, exactly how much the infected are considered the MIB is up for debate at this time.
4. Some combination of Jacob/MIB/and Island are all using Christian's form/body at various times for various reasons throughout the show.