I was just looking for the appropriate place to report this myself, and came across this post - just 10 minutes ago this issue was still there, so I'm not sure if it has been resolved.
I browsed to a portal page in my history, that told me my session had timed out after 20 minutes, then I clicked on the sign-in link on that page which took me to the login page that had another person's email addresses automatically fixed in the login box. I done it again to see if I'd done something wrong but it happened again with another person's email address. Clicking on the sign-in link down the left hand navigation worked as expected, allowing me to login as myself.