The problem is that right now there are four draft slots (Eagles, Chiefs, Bengals, Chargers) between GB and the first team that might be drafting a QB. Assuming that the competition would be to get a given quarterback before Cleveland, GB may be too far away. To get in a "in demand" position for those QB-wanting teams, they may have to trade down 2-3 spaces first.
Not that this scenario is beyond the range of possibilities. If Calvin Pryor or Jimmie Ward is on the board, the Bengals might want to leapfrog Philly (and of course the Packers) to get him. Or if Mosley is on the board, either Cincy or SD might want to jump ahead of Philly.
But just sitting at 21 isn't as likely to be in demand for a QB-seeking team.
The good thing for the Packers, ISTM, is that they are situated in a position where multiple positions might fall this or that way to be attractive. In addition to Mosley and Pryor (who a lot of people here would be upset if Ted Thompson traded away from, I expect), there are several CBs (DennardFuller, Verrett, Roby), a couple TEs (Niklas, S-J), OL (Martin, even Botonio), DE (Tuitt), that more than one team might have fallen in love with.
That, together with the plethora of 2nd/3rd/4th early talent, and with the variety of QBs available in the same way, is why I think a scenario of 2+ trades down is actually a real possibility.
Of course, given my predictive abilities, that means what'll actually happen is Ted causes massive heart attacks among Packer fans by doing a Lawrence Welk to get Sammy Watkins.
And do not be conformed to this world, but be transformed by the renewing of your mind, that you may prove what is that good and acceptable and perfect will of God.
Romans 12:2 (NKJV)