I know it's stupid, but I would feel really bad trading Nate. I think he really likes playing in Boston and knows his career was turned around by coming here (not statistically). He obviously hasn't quite lived up to expectations, but he played so well in November. Maybe he only plays well when there's a reason to be excited (playoffs, start of the season, etc).
With all that being said, I like Pietrus a lot. He is a very good defensive player with a long range shot. He would really fit the Posey role well.