Yes, you read that correctly.
Best Buy is going to be offering the 8GB iPhone 3GS with an instant rebate on December 10 that will reduce the price of the phone to... zero. The deal, initially reported on Boy Genius Report, is for one day only, and is valid for new customers, existing customers who want additional lines, and also to people who are qualified for an upgrade from an earlier iPhone. Reading the fine print, you need to sign up for a 2-year plan at the time of "purchase."
All stores apparently have a good inventory of the phones on hand for the promotion, which seems to be in line with AT&T's stated plan of trying to get as many iPhone users signed up by the end of 2010 in case the exclusive deal with Apple ends during 2011. The usual Best Buy price for the iPhone 3GS is $99, which is line with the pricing at both Apple and AT&T stores.
If the kids have been hounding you for their very own iPhones, this is a perfect opportunity to pick up the phones for nothing. Just be sure to let the kids pick up the costs for their own voice, text, and data plans. Bah humbug!