Today.
From what I can tell, the rumors of how they manage their daily stock are true. Whatever shipment they get today, they will hold for tomorrow. They make up tickets and each iPad available. Sometime before 9, they start handing those tickets out to people in line. At 9 their payments systems come up and they can begin selling to ticket-holders.
The tickets were gone well before 9. I barely got the model I wanted. If I'd been 10 minutes later showing up this morning, I wouldn't have gotten anything but an apology from Apple workers.
If you want one, I'd say get there on a morning well before 9. 8 seems safe. But as this information spreads, earlier than 8 is advised.