Hell, he could “escape” or something. Unless the Dharma folks know that someone can be “otherized” there’s probably no reason to suspect that he’d come back as a vengeful avenging weenie.
-Joe
But Ben DID know them. When the plane crashed, he already had a list of the people who would be on it. When he met them, he already knew their names just by seeing their faces.
He also knew that to fix the island jumping around in time, he had to get the the six back to the island. He wanted to bring all of them, because he wasn’t sure which ones would be instrumental in making the changes required to keep the continuity of time going. But he definitely knew they had to go back, and he knew where they would wind up. Ergo, he remembered them being there in his childhood.
What he didn’t know was that Sayid shot him in the past. That’s the only memory that needs to be erased.
No, he had Mikhail find out who was on the plane for him. And he got info from Ethan and by spying on them at the Pearl.
Also, when Ben gasses the Initiative, judging by his dialogue with his father, it doesn’t seem as if he’d been gone for an extended length of time in his childhood.
wonder if we’ll see that girl that young Ben was friends with in Dharmaville?
(remember her? she gave him a birthday present?)
Her name was Annie, and I certainly think that is a story yet to be told.
Yeah, I’m still pretty sure that she’s supposed to be the girl that Juliet reminds Ben of, not Juliet reminded him of 1977 Juliet. For a second I thought she might be Juliet and Sawyer’s daughter, but the timing is all wrong.
A silly little easter egg that a friend mentioned to me:
When Kate goes to the hotel to give up Aaron, Claire’s mother is staying in room K10. Kate mentions that she has the room two doors down, which must be room: K8 
I don’t even see that as really needing to be erased. Ben’s interactions with all of the characters in earlier seasons, including Sayid, can be easily explained – in that Ben is simply a Professional Liar. He is quite careful about what he reveals and what he doesn’t, when he’s talking to you. He only tells you what he wants you to hear.
We know that the writers never foresaw Henry Gale as being a reoccurring character, so there’s no reason to believe that, even if they did forsee the time traveling later in the story, that they wrote Gale/Ben so as to hint at having known the lostees in the past. So they are going to either have to go with a “Ben is such a good liar” thing (which honestly seems a teensy bit of a cheat, since even as a good liar, he should have let on clues for the audience to be in on something) or a memory wipe.
Luckily, Ben is such a cryptic, knowing jerk that his attitude towards the Lostees, especially about knowing all sorts of things that upset him, can at least somewhat be fit into the fact that he knows that they are going to screw with him in the past, and nothing can change any of that.