careful!
When my gaming group found a deck of many things, my barbarian drew a card from the deck of many things and had her alingment changed to Lawful Good. Now, since I had "asked for it" by drawing from said deck, I really had no room to complain - I just multiclassed her to Paladin and rolled with it. BUT if you drop in an item that forces an alignment change and the player doesn't even see it coming, you should be prepared for one upset player. I would suggest one of two things:
1. give the player a chance to avoid the alignment change. If the party runs across a +2 Sword of Becoming Evil put skulls all over the hilt and have the blade drip blood or something, anything to give some clue that says DON'T TOUCH! THIS IS EVIL!.
2. Make the alignment change gradual and make it a role-playing opportunity. If Reakk's Helm of Wickedness ends up on the head of Bob the fighter, have it mentally offer Bob temptations ("Oooooohh Bob, I'll give you he power to spit fire out your nose if you'd just stop being soooooo lawful all the time. I mean what are laws anyway. Why don't you take a step towards NG huh? That a boy. Hey! Whats so good about good anyway? Ever wanted to fly at will..."