They are doing this again in 2013! (I just called to verify, but check with your local Taco Bell!)
For Arizona residents, participating Taco Bells will give you three free tacos when you purchase a large drink, if the Diamondbacks score at least 6 runs. (A win or loss doesn't matter — well, for the tacos!) :) The deal is only good the day after the game from 4pm – 6pm. No coupon is necessary. Large drinks were $1.69 at my Taco Bell.
Here's a link to their schedule. It also lists the scores of past games, so you can see if they scored 6 runs!