My opinion has nothing to do with freeing up a spot for Pena, it has everything to do with Dunn signing a longterm deal in cincinnati. I don't think he ends up staying there longterm as I doubt they can afford the kind of longterm deal he will command and I'm not sure he wants to remain in Cincy either. Therefore, eventually they bite the bullet and trade him for some quality pitching in my opinion, especially considering their pitching staff.
I guess if he is a free agent in 07 they can wait until the next offseason to move him, but they run the risk of getting lesser offers if they wait too long.