I think that he stays as a SS as long as possible. His defense at SS has been improving every season. I do have to question the "weak arm" bit. His arm is not like Sano's but it is not weak by any means. I would call it "average". And there have been fine shortstops with average arms (e.g. Jeter) or even weak arms (e.g. Noarm Garciaparra) as long as they are accurate. The one thing that might make him move to 2B is not his arm, but his bulkiness, if it cuts into his range. He has gained considerable muscle weight and he looks more like Jhonny Peralta (another aveage arm All Star SS, btw) than Alexei Ramirez these days, but people thought that Cal Ripken was too big to play SS once upon a time. If he is anything close to Peralta, Twins fans would be very happy (and at this point Polanco has better plate discipline, ever as an A league player than Peralta ever had or will have, so I think that his ceiling is a little higher than that...)