American Airlines (AA, the company) has had More Room Throughout Coach for some years now. It really is a couple of inches more seat pitch.
However, they announced last year that the "more room" would disappear from their 757s and A300s. I recently noted that the "more room" symbol has entirely disppeared for all flights departing after 1st Jan 2005, no matter what aircraft.
Travel AA before they convert all their planes back to tight seating. That will probably take a year or two - pot luck until then.