Hi.
Prepurchase of the game will be available from April 10, https://buy.guildwars2.com/
Since noone have posted anything from the public beta (I have no idea if any NDA is stopping or if anyone just haven´t tried it, please share if you have), we can always hope that we will see something then. I plan to prepurchase and participate in the Beta Weekend events and promise to share what I can from that.