Initiate login from Terminal?
Show the login screen (without logging out):

/System/Library/CoreServices/Menu\ Extras/User.menu/Contents/Resources/CGSession -suspend


Switch current user (replace 501 with the correct UID):

/System/Library/CoreServices/Menu\ Extras/User.menu/Contents/Resources/CGSession -switchToUserID 501


(found at http://www.chipwreck.de/blog/2008/01/26/fast-user-switching-via-commandline/.)