Pre-purchasers with access to the headstart will be able to start playing on August 25th at 3:00 a.m. EDT, with the caveat that servers may be brought up three hours early to check stability one last time. Those who pre-ordered the game but didn't fully purchase a copy will have access to a one-day headstart beginning on August 27th at 3:00 a.m. EDT. If you're in either camp and want to get patched up ahead of time, you can download the client now and ensure that your computer is ready to catch every moment of the last two weeks before launch.
[Thanks to miattavich for the tip!]
[Update: The team has also posted the full server list for launch.]