I actually haven't heard the Black Widow argument, so I'd like to hear it.
As for my opinion, I think Loki is telling the truth that he isn't the traitor (though, it's true he did lie in that scene, claiming that he bears no malice towards Thor) simply because the truth, in this case, will only infuriate Thor more. He tells him a truth, which sounds far-fetched, so when Thor attempts to tell others about it, they don't believe the truth and it allows Loki to get away with it.
Also, the 'traitor', according to Loki, works for some of Loki's friends. Basically, since we do not know who's allied with Loki, we can only therefore look at those character that currently have superiors.
Indeed, if we choose to say that Loki may be lying about the concept that the traitor works for Loki's allies, then he could be lying completely about the existence of a traitor, therefore, we must assume (forgive the pun) that Loki's word, in this case, is gospel.
For example, Iron Man could be the traitor (note, he isn't available as an option). It would indeed surprise Thor, and perhaps Iron Man's been putting on an act. However, Iron Man does not appear to have any superiors (except stock holders) who would be associates of Loki.
Also, Pym appears to be toally devoid of superiors, and thus, out of the equation.
Wasp, like Hawkeye, Captain America, and Black Widow all work directly for Nick Fury and SHIELD and therefore could all be candidates for the role of traitor.
However, out of these three, the most obvious character would be Black Widow, and since Loki said the identity of the traitor would surprise Thor, it would be unlikely it is her.
Captain America I could not imagine to be the traitor, simply because it does not fit his character. It would be a cheap turning point to out him as the traitor.
Wasp could possibly be the traitor if we allow ourselves to go a little crazy and assume that she is the reason Hank is no longer part of the Ultimates. She got him kicked off the team way back in #6 by purposely sparking his temper to beat her, so he would get kicked off the team, become disreputable and at the same time get closer ties to Captain America whom, no doubt, her superiors would need. Her superiors are SHIELD of course.
Hawkeye, could also be the traitor, if one assumes that the reason he doesn't carry/operate his mobile phone when Nick Fury calls him (as seen in Ultimates 2 #4) and why he was late to the meeting in #2, is because he works for other superiors in SHIELD.
Indeed, in both the Wasp and Hawkeye case (and indeed, the case of any Ultimate who works directly for SHIELD) the superiors would be SHIELD generals we've not met. This would mean Fury is unaware of the traitor in the Ultimates - for if Fury is telling the traitor what to do... that would be "traitors" would it not?
As for the two mutants, Quicksilver and Scarlet Witch, they're superiors could indeed be SHIELD, or even their father, Magneto. But in any case, these two seem to be too far out of the loop of the Ultimates to be the traitor. They don't even seem to possess much of a motive. They could want to destroy the Ultimates, and infiltration and sabotage has been their method. This is a possibility, but it seems to me to reek of "these are minor character no one's really paying attention to, so we can do whatever we want to them". It just seems a bit cheap to me.
As for the Union - the Union is not part of the Ultimates and therefore, they are not eligible under Loki's promise that the traitor was in Thor's team - the Ultimates.
However, the Union also had a leak of information, involving Captain Italy. While they've not been connected in the book, I think that the same person is responsible and they seem to just be accelerating the timetable. If the same person is responsible, that person must also now have access to the Union. While this is pure speculation on my part, if this is the case, Hank Pym would seem even more unlikely since he has nothing to do with The Union (to our knowledge) while, it is slightly more feasbile that Wasp, Captain America, Hawkeye or Black Widow could all gain access to the Union's files (indeed, since Black Widow is from Russia, she would be the most likely culprit in this situation).
As for Banner outing himself, this is indeed a possibility - but I think it has one small oversight: It's stupid. It's the most stupid theory I ever heard.
In all seriousness, Banner has nothing to gain from his outing. Simply because he's now in more danger than when he was in his cell (and while we could say that Banner assumed that Hank would save him, it seems unlikely) since Banner was just under house arrest, now he his one of the most hated men in America and a fugitive. I suppose we could say Banner's intention was to be let off the hook... but it doesn't work for me.
Also, according to Loki, the traitor is still in the team. Banner isn't.
As for Thor being the traitor... he could be an unwitting traitor. He may not know that he's the traitor... but no, I'm sorry. I think it's ridiculous.
That leaves Fury. Nick Fury could be the traitor. But again, it does not seem to fit his character. It could be Nick Fury, is unaware of that there is a traitor, and that higher ups in SHIELD simply tell him his orders and he does them while the traitor works on their own within the Ultimates. On the other hand, Fury could indeed be the traitor, but not realise it himself. Fury could believe that he is actually doing good, by using the Ultimates to save the world from all kinds of threats, only to discover that he's been used and manipulated by Loki's allies. This to me, is the most likely suspect. Fury as the traitor (his superiors are SHIELD) but he himself, is most likely, unaware of the damage he is doing. It would certainly make Thor's parting words to Fury in Ultimates #4 more personal and more prophetic. "Oh, I am here to save the world, General Fury. Save it from people like you."
If we go solely on the clues we've been given so far, Fury seems to be the prime suspect. But as more clues come this will no doubt change.
I think #6 will give a big clue.