ArenaNet have announced that there will be a stress test from Noon – 4pm PST on Thursday August 2nd. See their post on Facebook for all the details.
[anet]We will be conducting a stress test on Thursday, August 2 from 12:00 Noon PACIFIC Time to 4:00 PM PACIFIC Time.
Unlike previous stress tests, we will be actively working on the game during the event, so you might experience connectivity problems or discover features that are not working as designed. Any issues you experience are a result of the rigorous conditions of the stress test, and are in no way representative of the state of the game at launch. By participating in this stress test, you’re helping us make Guild Wars 2 a better game. Thanks for your cooperation—we’ll see you in-game!