Liz the Squeak: I hate to disagree, but it absolutely is an American custom, or at least it is in the present form in which it is spreading in England. The witches/horror movie type costumes, the collecting of sweets after a knock on the door, the use of the phrase trick or treat, the whole thing is lifted directly from American movies. None the worse for that, and it fits very neatly into a long standing tradition of disguise, bad behaviour, and collecting money or food at this time of year, whether with or without performance of songs or plays (eg Antrobus Soulcakers". A generation back the local traditions(in the parts of England with whicjh I am familiar) tended to revolve around money-collecting with Guys, plus Mischief Night bad behaviour(generally on Nov 4, not Halloween.
    The changes since then have been almost exclusively American and media driven. Of course, all of these things link back to age-old customs. The songs, incidentally, seem to have vanished round here (Stoke), though the traditional phrase "spare a penny for the guy, mister" is still in use among the young children. But believe me, plastic tridents, bags for sweets and witches hats come from America.And why not?