I don't look for them to open the applications until after it goes live, or shortly beforehand. That way, all the previous beta testers will have priority for the client update and won't be hammered with the resulting bottleneck lag from the rush of anxious gamers.
Existing testers actually already have access to the open beta version of the client (see some other threads), so they're in the process of getting updated now, even though it sounds like none of them have gotten their open beta keys yet. Apps could conceivably open at any time, whether or not the selection process is involved at all (can always collect the info then hit a button later on to mass-mail keys if it's as simple as "apply, get key" or they /random on the back-end).
Since it's SE, it's hard to say what's going to happen, so we just need to keep our eyes out and apply whenever they end up opening up apps. :) (And hope to get a key emailed back.)