I think the last beta weekend before release has already passed.But I wouldn't be surprised if there's some 1 week trial or something.
I probably wasn't really clear there. By "pick it up now" I meant get GW2 when it drops. You'd be a bit behind from not playing GW1 or any of the betas to learn the UI and combat system and such, but not so much that you feel like everything is old content by the time you get your first taste of it. That's what happened to me with a lot of the TBC and WotLK raids, since I played about the first year of each, then quit for a few months and missed two tiers of progression.
EDIT: Guild Wars 2 Stress Test is today (Aug. 9th) from noon to 4 PM PST, but it's only open to people who have pre-purchased. So that's in 8 hours. Other than that, there's the head start on August 25th. Play for 3 days before release if you've pre-purchased, carry your character over.