Interesting idea... I could see this going either way... where you add 2 to the cart and the discounts module knows to decrease the price or where you simple add 1 and a B1G1 product feature updates the order details post-checkout. I'm curious how you'd plan on displaying this deal to the customer... like should the QTY in cart and checkout screens show the free one? Or should it just be a nice little message on screen somewhere?
Personally, I'd love to see this as a stand alone product feature for sites that don't need the whole discounts module.. but it would be nice to have it in both places.