The fine print of the ad, it noted that: "All restaurant managers decisions are final regarding to Free Taco offer."  Perhaps by giving each restaurant manager total discretion to construe the terms of the offer as they see fit, it operates as an 'escape clause' thereby making the promise of a free taco an "illusory promise".  If so, then no contract could exist for lack of consideration. 
 
I think this ad could give rise to a valid case of "promissory estoppel"/"detrimental reliance".  Clearly a promise was made that was conditional upon there being a stolen base in any game of the 2007 world series; and eligible persons who lined-up between 2-5pm on October 30th did so in reliance thereon, such that justice would require that Taco Bell live up its promise to provide a taco worth 77 cents or reimburse a customer for out-of-pocket expenses (money paid for gas?)