I found this interesting perspective on Halloween from the Greek Orthodox Archdiocese of Australia. I don't really agree with the argument's assertions, as it is not in favor of Halloween in any way, shape, or form apparently, but it does makes an interesting theological argument about it. This is something I have found lacking in contemporary American-Christian groups that are all about Halloween=evil=bad-Christians-who-celebrate-it mentality.
I take a more moderate approach to Halloween myself. Yes, I would agree that having little kids surround themselves with all the really gruesome, dark aspects of Halloween isn't perhaps the best use of our Christian teaching ability that we pass on to our children. However, I tend to split the difference and look at it as All Hallow's Eve, from which we get All Saints Day and All Souls Day.
My initial response to this, as I shared with my youth group, is that there is a deep, Christian symbolism in Halloween, both from the point of view that death is a reality that we don't try to deny, despite the fact that this modern culture tries to insulate us from it. Also, there is a great teaching moment from within Halloween that I always try to hit upon, with that being that Jesus triumphs over evil, death, and the grave.
While its fun to dress up as mummies, vampires, witches, or whatever, we ultimately serve a Lord greater than all those things. Halloween isn't the end, but only the beginning, as all the Saints always triumph in the end over all the evil in the world. I think as long as we keep that in perspective, its okay to go trick or treating, as our treat as Christians is a foretaste of that heavenly banquet prepared for us by our Lord, Jesus Christ.