Putting on my UI design apprentice hat.
James, is fast user switching going to a be a common task on a box with many user accounts? Corporate workstation? Small business file server?
Having a random workstation running an authorised session sounds like a security problem and highly unlikely. People in corporate environment will be locking their workstation or loging out after their session.
I am thinking fast user switching is strictly bound to a personal box. A family computer, a single box that is used by Mom, Dad & Son. That to me sounds like the most common case where fast user switching will be useful.
Now the question is wheather to rule out a case case of that many user accounts completely. It may well be, that a user wants to have two sessions running in parallel with a huge list of user accounts on the box.
Richie logs in as richie. Once the session is up and running he uses FUSA to “Log in as…” (“login screen” on your screenshot). Because the number of user accounts on the box is hight, FUSA only lists the currently loged in users.
Richie logs in as testaccount. Once the session is up, Richie can switch to the richie account.
Definitely start with the use scenarios. Otherwise you can drown in combinatory sea of corner cases or even never-happens-in-life cases and the UI will be overly complex.