Laid out over at ESPN.com, via NFL Network:

...if Favre takes 50 percent of total snaps with the Jets in 2008, the fourth rounder becomes a third-round pick. If he gets 70 percent of the snaps and the Jets make the playoffs, it becomes a second-round pick; and if he gets 80 percent of snaps and the Jets make the Super Bowl, it becomes a first-round pick.

So it would seem, barring a re-retirement or a snap of 'The Streak' (hey - crazier things have happened this offseason...) the Packers will be getting at least a third round pick.

UPDATED: Adam Schefter reports that if the Jets were to turn around and trade Favre to the Vikings, they would send their next three first round picks to the Packers.