See Weird Pop-Up Thread. Pop-up asks for your password, BUT this pop-up asked AGAIN for my password after I had already sucessfully logged in the regular way, and after I had posted and read other messages! DANGER, DANGER, Will Robinson!
I think we figured it out that a user had used an image that was previously uploaded to catsailor image database (user/login based). We were all being asked for our password so the system would show us the image. That user seems to have fixed his avatar link - I haven't seen it in a while. Either way, the request seems benign.