Ken Griffey Jr played 54 games in the minors as a 17 year old. Didn't hurt his career. When Harper actually signs with a team in August, 2010, he'll be 17 yrs. 10 mos. old. Griffey's first professional game, he was 17 yrs. 7 mos. old.
As long as the team that gets him (probably Washington) doesn't rush him as some kind of drawing card, I don't have any problem with this. It's out of the ordinary, but if he's being looked after by people that care for him and he's mature enough, I don't see what the big deal is.