To Melanie,
Forget about the previous Fast user script.
Use this.
http://dl.getdropbox.com/u/872430/Quicksilver%20Scripts/single%20user.scpt,
Instructions:
change the line in the script:
set username to "myusername"
to
set username to "whatever you username is"
Place it anywhere and run it from Quicksilver by bringing the script
in first pane & selecting action "run" from the second pane.
If you want to switch between multiple users frequently :
use this script:
http://dl.getdropbox.com/u/872430/Quicksilver%20Scripts/multiple%20users.scpt
Instructions
edit this line in the script
set usernames to {"username1", "username2", "username3"}