I just checked the flyer we got when on the Magic from Galveston Nov 2012, and the 20% special was just for cruises from Galveston through May 2013. It was a separate 1-page flyer that was sitting out along with what I assume is the usual booklet that contains the Onboard Booking Offer.
I'll be on the Magic cruise that I booked for 20% off in 3 weeks and will be booking again (re-shopping) for Thanksgiving 2013 on the Magic from Galveston, but I expect that it will be the standard 10%, though it would be a really nice surprise if they've extended the 20%!