Andy C. said:
Y'know, that might not be too far off. In one of the DC/Marvel crossovers, Joker makes a comment to Spider-Man about meeting once before, despite the previous crossover being retconned out and no one else remembering it. And I don't think it was a glitch in the writing, either; he's broken the fourth wall on numerous occasions (particularly in the animated series, where he'd make occasional jokes directly to the audience, and whistle his own theme music).
Every now and then they've hinted at Joker having a kind of "super-sanity" that lets him perceive more than what the average person should. However, I don't think they should ever give any definitive answers about the Joker. The fact that we don't know who he really is/was, or what he's got up his sleeve, or whether or not he knows about it, is a great part of the character. If Batman is the world's greatest detective, then Joker should be his greatest mystery.