A lot of people have been complaining that they can't read their questions once it's migrated to SU and while I agree that the password is no state secret (it's stated in the blog), most people don't have the means of looking for the password (especially not the SU population). They see private and then just get frustrated and leave.
I think as a simple solution would be to verify if the HTTP Referrer (or Referer as misspelled in the standard) is an SO or SF question, then simply don't ask for the password. (And of course don't ask the password on subsequent visits)