Cap sleeping with the enemy?
My first inclination was to rat out Hawkeye, but now I'm more inclined to think it's the Wasp. She already has a track record for deception (hiding the fact she's a mutant), she's a regular on the talk-show circuit (what better way to cover your tracks than by trying to be the high-profile hero?) and she seems to have some wild personality swings (hollering at Cap in the hospital, then flirting with him at a party). And don't forget the scientific jealousy she and Pym had toward Banner. Pym apparently got over this and wound up befriending and probably saving Banner. But the Wasp has the media connections (Fury pointed out her love of the limelight at one point), and there was no indication she ever felt anything but contempt for Banner.
The Widow and the mutants are, I agree, too obvious. Cap, Iron Man and Fury are too crucial as core characters in the UU. That leaves Hawkeye and the Wasp, and I'm leaning toward the latter.
There would be the added irony that Captain America has been unwittingly sleeping with the enemy. For those who want to see Cap knocked down a notch, that'd be a heckuva way to do it.