The Nats will be reluctant to move Johnson, who has a higher on-base/slugging percentage than the Mets' Carlos Delgado and is signed for $5.5 million in each of the next three seasons.
The Pirates, however, can tempt the pitching-thin Nats with their impressive stable of left-handed pitchers - starters Zach Duke, Paul Maholm and Tom Gorzelanny and relievers John Grabow, Damaso Marte and closer Mike Gonzalez. They could also trade an infielder to open a regular spot for Freddy Sanchez.
Who knows what Nationals GM Jim Bowden will be thinking, but even if the Pirates wanted Johnson, I doubt they could make this happen. The Pirates just can't trade Duke, Snell, or Gorzleanny - they only have four real starters right now as it is. After that, they have Shane Youman, Sean Burnett and scrap-heap-free-agents-to-be-named-later. There probably won't be any starting pitching prospects ready next year, either. That means that, without seriously reconfiguring the team, the only package I can think of that would even be tempting would involve Jose Castillo and a reliever, and I don't think the Nats would go for that.
Just for the record, though, I'd love to have Johnson. He'd be a great fit for the Bucs and for PNC.