First off, I can't see the Lions trading with some team in their own division. So I'd go after the Browns pick.
I think porky's hypothetical offer is more realistic pax, and going by that I WOULD do the trade.
I'd make one minor adjustment though, I'd offer a 2007 2nd rounder instead of the 2007 3rd rounder.
In return, in addition to the Browns #3 overall pick, I'd ask for QB Derek Anderson, and thus he'd be the QB that the Packers groom to replace Favre.
Johnson may have a Reggie Bush type impact, he'd sorely give us speed, size, and big play ability that we have been lacking.
A 3rd round pick would allow us to target a RB or a TE (one I like is Michael Allan from a D-II school).
So say we get a TE, then at least we bolster our depth, and Johnson is an immediate player. We are retooled to take one more legit shot at the SB with Brett.
I'd also pray that Packers can get a couple of good players who help out serving as backups in the second day of the draft.