I suspect this idea will get shot down from a LOT of directions, but here it is:
The Giants get the reigning AL MVP, and a guy who won it at age 24, at that. They greatly improve their middle infield, save a lot of salary and have control of Pedroia through 2015, whereas Cain can leave after 2011. They open up a spot for Madison Bumgarner -- or they don't think he's ready, Joey Martinez or Kevin Pucetas.
The Red Sox get a fabulous starter to replace the injured and seemingly not recovering Diasuke Matsuzaka, they get to move their shortstop Nick Green back to the second base position he is more familiar with, and they get an experienced shortstop in Renteria.
Would the Red Sox make the deal? Would the Giants? Should they? I'm leaving myself really exposed on this one. Have at it!