If you pre-ordered in full, and participated in a beta weekend (or were eligible to) then you can probably play on August 25th.

Obviously you won't be able to tralala into gamestop in order to get the discs early, but if you've previously played the beta, you can use that very same client (albeit an update patch will be necessary).

This is a note I got from someone at anet:

The client that you download from https://account.guildwars2.com/download (or the client that you used in the previous Beta Weekend Event, if you participated in it) will be the same client that you will use when the game launches. The client will be patched between now and then in preparation for launch, so be sure to update it often to minimize the amount of files that you will have to download when the game does officially launch.
Just thought I'd give a heads-up in case any of you have internet that is as total *** as mine.