Okay, I'm writing this up as a blog post to pull together a string of thoughts I had in another blog comment thread.
New Theory -- because Red Tornado wasn't the mole. This started as a joke and then it made more and more sense.
Kaldur is the mole.
Here's the deal. We know Kaldur is Black Manta's son because Greg Okayed that as a leaked secret. Suppose Kaldur has been an agent of the Light all along (either knowingly or as a "hidden personality sleeper agent" unaware of his mole-ness).
First of all, the Light always works through mis-direction. Whatever you think they're doing is only a cover for their REAL plan -- like the prison break that was really to get Hugo Strange put in charge of Belle Reve.
So if Kaldur is the mole, suddenly Sportsmaster's "slip" about there being a mole makes perfect sense. Sportsmaster made the comment blatantly where Roy Harper/Red Arrow could HEAR it. Aqualad talks to him afterwards and "has to investigate" (which he does with comic stupidity "anybody having trouble with their homework or ... loyalty???") Aqualad reports to Red Tornado and when Batman and the League eventually investigate an apparent leak or suspect a traitor on the team, Kaldur will be PROVEN innocent because he is the one who REPORTED a traitor.
And if we need further PROOF that Kaldur is no traitor, why he put on the Fate helmet and became Dr. Fate!!! Surely that proves it because Nabu the Lord of Order would detect evil or deception in anyone who put on the helmet.
BUT DID HE PUT ON THE HELMET???
We SAW him put on something that looked like the Fate Helmet and he sure SEEMED to become Dr. Fate... BUT DID HE? WOTAN was there throwing magic all over the place -- could he have made it LOOK like Kaldur became Fate but didn't? It sure explains why Kaldur (who is apparently a perfect candidate to be the next Fate) had no problem taking OFF the Fate helmet. "He almost didn't let me go, but your friend Kent helped persuade him". Doesn't that seem a bit fishy in retrospect?
The bottom line is, at this point NOBODY suspects Kaldur. He is unquestionably "the one you least suspect" and as Greg has said, everybody is going to be surprised when the find out who the mole is.
P.S. CONVENIENTLY Kaldur is the only one of the team who does not Mind-meld in Bereft -- was he REALLY knocked unconscious by Psimon??? How come he's the only one who doesn't wake up -- remember, Megan was needed to get him to the Bio-ship because he was dying -- to save him they had to abandon Superboy (aka "the Weapon") and only Miss Martian's disobedience and stubborn attachment to Superboy defeated that scheme. Or was he faking all along?