Using my b'day...if I went on 10/12 and bought a new AP, I would indeed be able to use it on 10/12/09. (our APs from last September were dated 9/18/07, and we could have used them on 9/18/08, for example...you get an *entire* year)
Because it would be my b'day but I didn't need admission, I would have two choices, according to the rules of the promo. I could choose a Birthday Fun Card which would be in the amount of a one-day, one-park ticket, good on certain merchandise (no food or booze) at certain places on THAT day and that day only. Or I could snag a one-day one-park ticket good through the year until 10/12/10 (not sure if it would be good ON the 12th or if it would expire on the 11th?), but that ticket is not upgradeable, per the info pages.
I'd be going for the Birthday Fun Card.