They announced the prize in an Insider post:
It’s been a spirited competition, with much brewing and duelling but we would like to congratulate Gryffindor as the victor of the Pottermore House Cup for November, 2012.
Commiserations to Slytherin, Hufflepuff and Ravenclaw, but your chance to make the Pottermore House Cup yours will come again – the house points have already been reset for the next contest in the Great Hall.
As a reward for winning the Pottermore House Cup, Gryffindor will be able to download beautiful artwork of their common room as wallpapers and screensavers for their electronic devices.
Members of Gryffindor house, keep an eye out on your owl notifications for further details when these become available, they should be arriving soon.
Until the next Pottermore House Cup, we wish everyone good luck and keep up the great work on your potions and spells.
This prize will come as a surprise for many, as no one expected artwork to be the prize of winning the House Cup. A lot of people thought it would be new Prisoner of Azkaban chapters, and I for one thought it would be the upcoming Games Hub (the last padlock on the top of your Pottermore screen). In the meantime, we can speculate what Prisoner of Azkaban and the Games Hub will include.
All Gryffindors should keep an eye on their notifications over the next couple of weeks to see if they arrive. Once they are available, we will share them with you on here.
To all of our US readers, Happy Thanksgiving! Enjoy the time with family and friends and eat lots of turkey! 🙂