At PAX Prime this year, Bioware announced at their panel for Star Wars: The Old Republic panel that they will be sending out a horde of beta invites to hopefully tide some lucky fans until the games release. Bioware has stated, they will begin sending out the invites starting September 2, so keep a close watch on your inbox (and spam) as they are sending them out periodically throughout all of next month.
This being the same game that sold out within 4 hours across Canada when the special edition was announced so you can expect a lot of competition for these spots. As they are limited, keep your lightsabers down, no need to turn to the dark side.