I think a couple of years down the road, folks won't be debating whether he's a PG or SG. He may not reach Wade's pinnacle of success, but he'll simply be one of those guys that you want on the floor. Our pal Dwyane Wade spent ample minutes effectively running the PG spot early in his career, usually leading his teams in assists. He wasn't really accused of being a prototypical PG - it's just that his team's played better when he was in the game.
Maybe we're starting to see this out of Shumpert. At PG, he's a size/length mismatch - his height allows him to see the floor. He's uncommonly quick, allowing him to keep up with the little fast guys in spite of his size. By virtue of being bigger, he doesn't need to be as tricky or creative to be effective - but if he does have that wrinkle to his game, then we're really in for a treat.
At SG, he's just about the right size - but he's quicker and more athletic than most SGs, allowing him to slice by them and heck, jump over them. But his ball handling and passing skills also probably exceed most SGs, so he becomes at least a strong double threat.
The key question about his game lies in his shooting ability. The Knicks staff seemed convinced that his form is sound - it's a matter of curbing the off-balance shots. Sure he's capable of making some of them, but like the great players in the NBA, he'll need to learn when to roll the dice on a tough shot or when to pass up the opportunity to get the team a better shot (whether getting the ball to a teammate or setting up all over again).
In any case, we're probably looking at someone along the lines of a young Michael Finley or Sprewell...