#1 - Aug. 1, 2013, 5:28 p.m.
http://us.battle.net/hearthstone/en/blog/10529909/the-key-to-the-hearthstone-arena-8-1-2013#best
This might cause some confusion though...
The deck you made before leaving will be the deck you use until you lose 3 games or win 9 games. Fight hard!
It was stated in the stream that you could keep playing past 9 games, but the reward will not change. This statement on the news page seems to imply that your deck is auto-retired at the 9th win.