Well, let's face it, Favre is probably better than any of the three QBs on the roster. He's definitely on the decline, but he could give us a decent season to protect the young QBs and train 'em up with his experience.
I could see the Packers being interested in trading him. Out of division, out of conference, out of mind. As for Favre, he probably won't like it, but honestly, the Ravens are a better team than most of his realistic options. Most of the teams in need of a starting QB are not winning teams. Ravens have a good running game and a good defense and more of a chance for Favre to start winning now than say, Chicago or Miami.
That said, I doubt it happens. For one, the Ravens coaches really believe in their young QBs and that one of them will step up and be the player they need, which isn't a superstar, but a game manager. He doesn't have to be Brett Favre, he just needs to be Trent Dilfer.
Second, the cap hit would be tremendous. According to league rules, even if Favre restructured his contract, the team he's traded to needs to be able to get him in under the salary cap with his initial salary. With all the expiring contracts coming up, I don't see this happening.
But, if we start hearing about the Ravens actively pursuing Favre, then you can bet it's because the coaches have lost confidence in their QBs. So I'd say keep an eye on the situation at least into the middle of training camp.