I have the same issue. I've been playing minecraft since alpha and have yet to experience this problem until yesterday. I would also appreciate some help on this matter. I've tried multiple browsers and still get the same message.
Same problem here. Would this have something to do with trying to play the game on multiple computers? I played Minecraft on my work computer for the first time this weekend and when I got back home I attempted to play it on my own laptop, but I started getting this message.
My work computer doesn't remember passwords and no one else there has even heard of Minecraft, so it's not possible that someone else has gotten on the computer and started playing under my account while I'm at home. Any hints as to what might be up?