Announced on the official forums:

We want you to experience Guild Wars 2 and Lost Shores with your friends, so on Monday November 12, we will turn on Refer a Friend Free Trial. You will be able to invite up to 3 Friends to play Guild Wars 2 with you for free from November 15-18.

To do so, you need a Guild Wars 2 account in good standing that was created before 23:59 PST (7:59 GMT) on November 6.

We will give you more details once the program goes live, but start thinking about who you want to invite!
So if you have any friends who haven't bought GW2 yet because they want to try out the game before buying it, get them ready for their 4-day trial period!