According to wikipedia it originally was a "cold water challenge".
I've heard that it started with pro golfers just daring each other to do something or donate to the charity of their choice. ALS charity ran with it and it became ALS-centric.
Either way good cause I guess. I've seen versions where you either do the ice bucket thing OR donate money, and others where you donate $10 and do the ice bucket thing OR donate $100. It seems kind of silly that a bunch of people are just dumping ice water on their head to avoid giving to charity, but whatever makes 'em happy and some good obviously comes from it. I'm going to do it myself labor day weekend...holding off for two weeks, and it'll be a convienent way to empty the coolers after my daughter's 3rd bday party. IMO it'd be silly to buy ice to avoid donating to a charity...