SWA has not updated their Travel Tools page with the next release date, but has updated their FAQ. In the FAQ they say:
Link to SWA FAQ
Of course the date is subject to change (and seems perhaps more likely to change when there are discrepancies between the Travel Tools page and the FAQ).
19. How far in advance may I purchase Southwest Airlines travel?
You may now purchase Southwest Airlines travel through March 11, 2011. On October 5, we will open our schedule for sale through June 3, 2011.
Link to SWA FAQ
Of course the date is subject to change (and seems perhaps more likely to change when there are discrepancies between the Travel Tools page and the FAQ).