Ben is very firm on the concept of player/character separation. There are certain things that he, as a player, knows, but that Obi-Wan, the character, cannot know given the progress of the story so far. And if Ben knows something but Obi-Wan doesn't, he's not sharing it with the other characters, no matter how important it is.
Of course to some roleplayers - <cough>Pete<cough> - maintaining this sort of faithful separation is not only silly but tantamount to character suicide. Player knowledge is an important tool to be used to maximise the effectiveness and success of your character.
On the other hand, despite Pete knowing this vital piece of information all along, he figured it was superfluous to the more important task of getting cool weapons and generally causing mayhem on a desert world that they'd never be returning to.
Jar Jar: Senator Palpatine still not talking to mesa, Your Highnessness.
Queen Amidala: Yes, I'm not sure why, Jar Jar. I asked him about the Orb. He thinks the Trade Federation have been using it to power their orbital corporate headquarters at Naboo.
Jar Jar: Oooh. Mesa has to go back to Naboo.
Qui-Gon: Wait. The Trade Federation have the Orb?
Jar Jar: Wesa already knew that.
Qui-Gon: We did?
Jar Jar: Padmé told wesa on Tatooine!
Qui-Gon: Why didn't anyone say anything?
Jar Jar: Mesa be thinking yousa had a plan.
Obi-Wan: And I wasn't there at the time.
Qui-Gon: You knew too?!
Obi-Wan: Not in character...
R2-D2: Oh, you took player/character separation as a flaw, huh?
Obi-Wan: That's not in the rules...
R2-D2: I meant in real life.