Of the 16 current contenders, few are in need of first base help, which should reduce the demand for Pena. The eight AL contenders generally don't need designated hitters, so that route probably would not help Cubs GM Jim Hendry.
The Pirates and Diamondbacks strike me as two teams that would benefit from renting the Pena. The D'Backs would might prefer to turn to Brandon Allen or Paul Goldschmidt at first, leaving the Pirates as the lone suitor if they wish to improve upon Lyle Overbay. The Bucs make for an interesting match, as typically it's been the Cubs swiping useful players from them. I also like the fit because the Pirates could avoid giving up any useful young players for Pena, instead offering to relieve the Cubs of most of the slugger's remaining $6.6MM.