The information was released originally by the Bob McGinn on the Milwaukee Journal Sentinel's Packer Insider pay-for-content part of their website. ProFootballTalk.com then leaked the information to the rest of the public.
Mike Florio of PFT writes, "At one point, McGinn says that the Packers made a trade proposal to the Ravens, which was rejected. As we hear it, however, the Packers called the Ravens only to gauge their interest, and were told that the Ravens weren’t interested."
The article goes on to list teams possibly interested in Favre including the Chiefs, Bills, Dolphins, Jets, Panthers, Buccaneers, Falcons, Redskins, Vikings, Bears, and Lions.
The rumors regarding the talks with the Ravens included a third round draft choice and a player for Favre. The Ravens reportedly weren't interested. It would be interesting to speculate on which player the Packers were interested in.
It would seem that after both Jason Taylor and Jereremy Shockey netted second round picks in addition to some later round picks that the Packers could command more than that. If they were to get a third round choice, it makes sense that the player included would be one that would contribute immediately.