There's a recent article on nfl.com as well as other sites, see http://blogs.nfl.com/2012/06/20/shocked-vikings-players-and-coaches-react-to-the-news-that-wr-percy-harvin-wants-out/ that describes coaches and team mates reaction.
Some fans go so far as to describe Harvin as whiny, you could say he's spoiled. He came from Florida where they won a National Championship, then in his first year with NFL he get's the gunslinger QB Favre and a MIN team that had a good chance to make it to the playoffs and had Super Bowl aspirations.
Now my guess is he had a reality check moment at practice (mini-OTA) and he knows this team is nowhere close to competing at getting into playoffs, much less the Super Bowl. There division maybe the toughest in the NFL with Green Bay, Detroit and Chicago having QB's in or near their prime. Who does MIN have as a QB? Can you name him off the top of your head? And remember the adage "QB's make the WR's", well Harvin wants to win like everyone else, he just realizes that the chances of winning consistently are minimal at best and thus want's out due to QB most likely not up to par with the rest of division.
I can't blame Harvin for wanting out, most NFL careers are fairly short and if he sticks with this years team in the near term his stats will be unproductive due to QB and his value will diminish the longer he stays with team and present offensive scheme. The question should be "Will management let him go?" Remember last year wasn't a productive year for him as far as stats are concerned, but this can be attributed to poor QB play and since MIN didn't upgrade at that position this offseason, he'll most likely have a similiar year as last year at best. He just wants the taste of success again and is finding it hard to swallow the bitter pill of losing and ineffectiveness that I suspect will permeate the team this upcoming year. MIN is years away from contending and Harvin wants to play for/with a contender, not a pretender.
If I were management I would trade him while their still is value, if they wait his value will diminish throughout the year and at the end of the season they'll most likely cut him and he'll be picked up by another team. This would be a disaster scenario for MIN, but it appears DET gave MIN their management curse and I predict MIN will hold on to Harvin for too long.