Former Giants RB Tiki Barber isn't joking, he's unretiring and returning to the NFL at age 35. This isn't a discussion whether he should play for the Green Bay Packers. This is just a bad idea all around. I guess it's his choice if he wants to take the chance.
This reminds me of the comeback attempt by former Packers WR Robert Brooks. Barber retired while he was still a starting running back, and Brooks too retired while he was still a starter after the 1998 season. I think Brooks had even signed a new contract before the season, and the team tried to talk him out of retiring. While Barber has had some well publicized financial (and personal) troubles, Brooks (I think) had been hit hard financially by some bad investments. He played for the Broncos in 2000, and that didn't go well. He was retired again after the 2000 season. You could probably insert the failed comeback attempt by many former starters here.
It was sad when Brooks's comeback attempt didn't work out. While Barber hasn't had the injury problems like Brooks, I hope it doesn't end the same way for him.