Early in the 2008 season, Palmer partially tore a ligament in his throwing elbow—ending his 2008 season. He came back in to lead the Bengals to a division title in 2009, but he no longer displayed the cannon he had early in his career.
Palmer opted to rest and rehab that elbow over Tommy John surgery, and injuries like that usually take a couple of years to heal completely. His arm strength did improve in 2010, but it still wasn't quite what it had been when Palmer first entered the NFL.
However, it has since been confirmed that Palmer's arm strength is back by wide receiver T.J. Houshmandzadeh, who catches passes from Palmer every offseason. He said in a phone interview on NFL Network, "He's healthy. He's throwing like the Carson Palmer of old."
That was before Houshmandzadeh was signed by the Raiders.
Campbell is throwing now but not very far—there just isn't enough time to get him back to where he was early this season. It's one thing for a bone to heal, and another thing for the atrophied muscles of his throwing arm to be strong enough.
There's no way that we would see the same Campbell this season.