If you're interested in trying for one of the 3,000 BlizzCon tickets being sold by lottery, and you had a Blizzard Account before the evening of August 12th, head over to the Blizzard Store now, as Opt-ins are open.
You should able to put yourself in to the drawing on the Account Management page. The opt-in period will last until the evening of Sunday, August 24th, and you will have an equal chance of getting a ticket no matter when you sign up.
After the Opt-in period is over, 1,500 accounts will be chosen at random for a chance to buy up to 2 tickets. Be sure your contact information is up to date so they can contact if you win. Good Luck, and we'll see you at BlizzCon!
[Thanks for the forward, Jeff!]