You state that last year was a best case scenario for a second ring, but the reasons you list (Solid defense, great talent on offense) aren't going to change next year - if anything, the experience gained from the '07 season may enhance their chances in '08.
The Packers look like a team on the rise, and were the youngest team in the league last year. Almost nobody picked them to even make the playoffs, yet Brett led them to a 13-3 record and NFC homefield advantage in the playoffs. Saying he wasn't a difference maker might only be focusing on the things you choose to focus on.
Does the following look like any windows of opportunity are closing regarding a 2nd Super Bowl ring?
http://www.packers.com/news/releases/2008/01/02/3/