Hmm, sounds like it'll be like the October 2004 open beta weekend where all you had to do was download the game client from guildwars.com and pick an e-mail address for your "account" for the weekend.
While it does sound cool that now we can get all of our friends to try Nightfall for free, I hope the weekend does not suck like the Factions PVP Weekend where anyone could download the client and try it. That weekend was awful with so many people spamming insults and harassing everyone; if someone was being a tool and got their account banned, all they would have to do is make up a new e-mail address and they had a new account. Bleh.
Still, nice to finally know the date of the PVE weekend, can't wait to try out Nightfall