That would be an act of pure desperation, and if I were a Vikings fan I would be pissed.
I'm looking at the available (or possibly available) QBs, and there's no way I'm going for Favre unless I'm dead-on positive that will mean a Super Bowl win - not just a berth, a WIN. Otherwise, it's a perfect way to piss a good team down the drain.
What they need to do is either get someone like Cassel, or settle down and let T-Jack develop. Or at least get someone like McNabb who's going to be good for a couple-three years.
That said, I wouldn't be shocked if it happened, nor would I be shocked if it didn't work.