f9-alpha logins dead

["David G. Mackay" <mackay_d bellsouth net>]

I just updated an f9-alpha installation, and now I can't log in to X.  I
see a message from gdm-simple-slave saying that it's "unable to
determine session to activate" and then it restarts the login process.
>From the user standpoint, I give it a username and password, the screen
goes blank for a while, then I go back to the gdm login screen.

Dave