Plouffe is making the transition to the OF in his age 26 season. He was a 1st round draft pick (20th overall) by the Twins in the 2004 amateur draft out of Crespi Carmelite High School in Encino, CA. Making a transition from a middle infielder to an outfielder relatively early in someone's career is not a rare phenomenon. A good comparable is the player who Plouffe is most likely to replace in 2012, another former Twins' first round pick, Michael Cuddyer. Michael Cuddyer spent 2004, his age 25 season splitting duties between second base and RF. His numbers at 2B were .206/.225/.309 (.534 OPS, .103 isoP) and at RF: .278/.359/.444 (.803 OPS, .166 isoP); very similar splits as Plouffe. In 2005, (his age 26 season) Cuddyer hit at 2B: .190/.370/.190 (.561 OPS) at 3B: .255/.317/.429 (.746 OPS, .174 isoP) at RF: .318/.366/.455 (.821 OPS, .147 isoP). When he moved to RF full time in 2006, (his age 27 season) he hit .274/.353/.470 (.823 OPS, .196 isoP.) Note that Terry Ryan was also the person who moved Cuddyer to the OF. I think that Plouffe can be a valid replacement for Cuddyer in the 2012 season and there is a good possibility that he will provide the team with right hand power that has been lacking.
A few other notable outfielders who made the switch from the infield in early ages (season and age of the switch in parenthesis) :Jose Bautista (2007, age 26), Ryan Braun (2007, age 23), Alex Gordon (2009, age 25), BJ Upton (2007, age 22), Michael Morse (2010, age 28), Emilio Bonifacio (2011, age 26) and Alfonso Soriano (2006, age 30.) So there is precedence, there are positive indications of future performance, thus is a trade well made by Ryan and at a very low cost.