I suggest in the password panel if you registered through another site like facebook it would say "You registered through an external service and didn't enter a password. How about entering one now?" That way if you disambiguate your facebook account you can still log in.
You can't just enter a password though, otherwise if you stayed logged in (and you would if you leave your self logged into FB; it's part of their single sign on stuff) and I came to the forums as you, I could hijack your account. I'd like to be able to add a password check with disassociation, but I can't... If I could, then I could just let you set a password directly.