This theory falls on the heels of my previous theory that Flocke and the John Locke we have seen all along are one and the same.
Locke is the reincarnation of MIB. In other words, same soul, new body. If you grant that reincarnation is at least possible, you will grant that there are a select few who actually remember their previous incarnations.
The puzzle pieces that point to this follow:
1) Richard Alpert's "test" - the one he gave to young John Locke while he was living in a foster home. This is clearly a test to see if John is the current incarnation of an already existing entity. The fact that John didn't "pass" the test in Richard's view may have more to do with a weakness in Richard than in John.
2) John Locke saying "in a manner of speaking", in response to Ben's asking "Have you two met before?" - just before killing Jacob.
3) Canton-Rainier is written on the side of the carpet cleaning van that is toting John's corpse around, and, as we all know, Canton Rainier is an anagram of reincarnation.