I tend to agree with Tax-chick. If you took that "dark side of the force" business seriously, it would undermine basic Christian ideas about the nature of God and of good and evil. But I believe most people laugh at it.
The same with Harry Potter. People dress up as witches and wizards to go to the opening, but they do it in the spirit of fun and games, not some dark yearning to practice witchcraft. Kids wield light sabers in the same spirit of fun.
I think parents have to be discerning. Some children and teens are spiritually unstable, and reading Harry Potter books may lead them into investigating ritual magic seriously. (Someone pointed out on another thread that bookstores are marketing books on witchcraft and even Satanism in conjunction with the H.P. blitz. I noticed some of this myself when the new editions of LOTR were issued, in conjunction with the films.) In my opinion, a young person in this situation should be distracted from any fantasy or magic-related literature, not just Harry Potter.
For me, the point where any fiction becomes a spiritual danger is when the reader wants it to be non-fiction.
You should see my 3-year-old and 18-month-old waving paint sticks at one another, going "ZZZZZ! ZZZZZ! ZZZZZOT!"