That all depends on his role. Stealing the cables is illegal, but publishing them isn't. That, in short, is why Bradley Manning is in jail, and the editor of the New York Times is not. The question is, which role did Assange play?
If Bradley Manning planned the theft of the cables himself and then handed it off to Wikileaks, Assange is in the clear, just like the New York Times. But if Assange and Manning had a dialogue, and Assange guided or helped Manning in any way, Assange is guilty of espionage.
I think that if the U.S. had a strong case, Assange wouldn't be hanging out in the Ecuadorian embassy, instead the British would have rounded him up and sent him off the the U.S. for trial a long time ago. But Assange isn't stupid or complacent, he's smart and paranoid. So he did one of two things. Either (a) he helped Manning, but he did so in a way that was completely untraceable, or (b) he was smart about it, and said "whoa, hey dude, happy to help distribute this stuff but I'm not going to be involved in stealing anything."
I'd bet that Assange kept his hands clean. He's expecting the U.S. to come after him, and so if someone approaches him about the possibility of stealing American intelligence, he'll suspect a trap. Even once he's satisfied that it's not a trap, he'd see the risks posed by direct involvement. Another thing to keep in mind is that the U.S. has been leaning on Manning for a long time. You can bet the interrogators and prosecutors have told him that if he implicates Assange, they can get him a better deal. So Manning has told them Assange isn't involved- and either he's steel-willed and won't break, or he's being honest. Either way, the U.S. is screwed.