Please don't think that what children see doesn't effect them, and deeply. Visuals effect adults, we all know that. Even more so for children.
AMEN Brother!!! That is my main concern with Harry Potter as well, that it opens a door of interest to that which most of the debaters on this thread agree is forbidden in scripture...witchcraft, the occult, etc. The bulletin board you mention in your post is an excellent example; I've also read that book stores have moved books on witchcraft and the occult to the front of their stores due to the tremendous increase in interest in those subjects as a result of the Harry Potter books/movies. I believe that there are many Harry Potter "toys" this year that will encourage kids to "play" with witchcraft and the occult.
Face it, folks. It is common sense that just because a book or movie is fiction doesn't mean it can't sway an impressionable and/or young mind to try something in the occult, "just for fun". For many, maybe even most, Harry Potter will only be a fun escape...for some, Harry Potter will lead them to dabble where the Lord clearly told us not to dabble. I won't take that gamble with my kids; if you want to go ahead.