I have come accross this twice now on a Windows XP machine and a Windows Vista machine both running latest IE7. If I sign in as me on someone elses machine while they are logged on as them (as I have more rights) I cannnot get it to stop signing in as me again. Tried sign off, restart IE, reboot, clear cache, delete cookies everything. After a while the Vista machine rectified itself but now over a week later the XP machine still signs into SharePoint as me. Even though email, Communicator, the pc are all logged in as the other person.
How on earth can this be fixed!