PRINCE FIELDER: .927 career OPS (tied for 13th among active players, ahead of Ortiz, Teixeira, Hanley and David Wright. He's 21st among active players in career OBP (ahead of Dunn, Utley, Posada and Ichiro) and 11th in active SLG ahead of Matt Holliday, Ken Griffey Jr, Mark Teixeira, Chipper Jones. That's pretty impressive at age 26.
He's another David Ortiz in many ways, just better at an earlier age. I would take a chance on moving Beltre and shifting Youk to 3B for the remainder of the season if it meant getting Fielder for an extra post-season run--unlikely to happen but I'd be fine with them moving Kelly to make it happen, as I like what I have seen from Doubront and think they can scrape together a #5-6-7 starter without too much difficulty as needed in 2011, 2012, 2013. The DH spot will probably be available next year.
At the worst the Sox would lose Ortiz in 2011 and would have moved Beltre so they would need a 3B again in 2011. I think many of us prefer Youkilis at 1B and think he serves the team best at that position, certainily more than Prince Fielder or David Ortiz would. DH could be primarily Fielder's, but he could occasionally play 1B so guys like Drew and possibly Martinez can slot in there over the next couple seasons.
It would take serious balls for Theo to move Beltre from this lineup, but that move would benefit the club immediately and longterm. Fielder is a good enough hitter to build a team around, especially at his age. The Yankees are unlikely to pursue him so there's a good chance he's on Boston's radar.