Topic Change Your Password, Right Now
For those who don't frequent the general forums, this is an element of concern, and you should learn about it now, not days from now:
The thing to stress here is that this is a security breach on Blizzard's end of the equation - it has the potential to affect you regardless of how complex your password is, how recently you've already changed it, how safely you browse, how commonly you check for virii, etc. This isn't one of those "Have I clicked a suspicious email lately..." things. Take their advice, do yourself and your guildmates a favor, and go change your password right now.
And then tell everyone you know and love to do the same.
My new password is 27 characters long, contains three prime numbers, a palindrome, directions to Bolton, and a joke.
A friendly reminder to change yours if you haven't yet, and tell everyone in your guild to do the same, especially as people who've been away and inactive for ages start coming back to the game as they catch the scent of New Expansion in the air.