We were just at SeaWorld on May 9. As you exit the park, there is a sign and ticket window on the right that says to turn in your ticket for a second day free (if used within 7 days of your original visit). All you do is turn in your admission ticket for that day and they take your picture and print you a new ticket.
You can also use that window to upgrade your single day admission to the ticket that allows access to Busch Gardens or to a SeaWorld passport (annual pass).
I haven't seen anything in writing about the second day free offer, but they seem to have made a permanent accomodation for it. However, I guess since they don't have anything in printing (at least that I have seen), that the 2nd day free offer could end at any time.