As part of ensuring Guild Wars 2 (GW2) player accounts remain secure, ArenaNet is enforcing a mandatory password change for current GW2 accounts starting on February 7th, 2013. Players that chose a password prior to ArenaNetÂs implementation of the blacklist consisting of compromised email and password combinations known to have leaked to the internet, players will be unable to select a password on that is on the list or that was previously used on the GW2 account.
You can make the change early by logging into your account and selecting a new one. The password changes will be rolled out over time, so you may or may not be prompted for a reset on February 7th. My best advice, go ahead and get it changed and avoid the hassle.
Source: GW2 Password Change Notice