This trade SHOULD HAPPEN.  I see no reason why SF should keep Cain at the expense of getting FIELDER.  I'll tell you why.

Cain is a losing pitcher.  Except for his first two seasons, in which his record was 1 game above being a .500 pitcher, his ENTIRE CAREER has been in losing seasons.  This season alone, he has given up more HITS, more RUNS, more HOME RUNS, and WALKS than any other season of his career.  He has the most HIT BATSMEN, most INTENTIONAL WALKS, and is tied for the most BALKS in his career.  Also look at his COMPARABLE TO AGE player, Moe Drabowsky.  Another promising rookie, he came up when he was 20 and was a starter.  Unfortunately, like Cain he was a LOSER and he had his first winning season from the bullpen at age 31.  Matt Cain is signed cheaply for a few more years, but after that do you want to pay him BIG BUCKS to be in the bullpen?

Prince Fielder, on the other hand, is a winner.  He's in the playoffs this year, Cain has yet to be in the playoffs.  He's the youngers player to hit 50 home runs.  50 HOME RUNS.  He would have lead the Giants in home runs this year.  Even though he only hit 34 this year, his average is a bit down so when he hits for a higher average next year he'll hit more home runs.  He tied his career high in TRIPLES this year, and even has 10 SACRIFICE FLIES, showing he will give up his own better average just to make an out for the team.  Fielder is also a good fielder (get it?).  He has a .990 career FIELDING PERCENTAGE and made the most PUT OUTS of his career last year.  Also, Prince Fielder's COMPARABLE TO AGE players include Lou Gehrig, Willie McCovey, Will Clark, and Willie Mays.  Are you going to not trade for them?

I really think this is an easy trade to make, a losing pitcher, for a winning hitter.