I doubt an NFL team hasn't considered trading the top overall pick in my lifetime. "We're open to all options with that pick," said Detroit GM Martin Mayhew, just as long as we get a boat load of picks in return, I'm sure.
San Diego has started a tradition of trading away the 1st overall pick, in 2001 for QB Michael Vick and in 2004 for QB Eli Manning. They made out better on the deal for Manning by receiving a 1st round pick in 2005 in addition to the 4th overall pick in 2004 (plus two other picks).
I do not expect the Packers will trade with Detroit, and in no way would I want them to give up three or four picks for the top pick. It would probably cost the 9th overall this season, their 1st round pick next season, and one of their 3rd round picks this season. That price is lower than what San Diego received in 2004, but Georgia's QB Matthew Stafford isn't as highly valued as Vick or Manning were.
But even if the Packers would consider moving up, who the heck would they move up to draft? Not Stafford. Would they want one of the top offensive lineman? I have no idea who they would actually move up and draft.