To show the login screen, install the Extra Scripts plugin. It
includes a script called Fast Logout. It is a misnomer, since you
will still be logged in, but it will switch to the login screen just
like Fast User Switching will let you do.
Leopard seems to have broken the User Accounts plugin. Until someone
knowledgeable fixes it, i've been using te following Applescript
instead:
do shell script "/System/Library/CoreServices/'Menu Extras'/User.menu/
Contents/Resources/CGSession -switchToUserID 502"
You will need a copy for each user on your machine. Modify the 502 to
the appropriate user id number for each user. In general, the first
user created on your Mac has the uid 501, the second is 502, etc.
Save each copy as something like "Switch to <username>.scpt" in ~/
Library/Scripts, and make sure that folder is in your QS catalogue.
If you want to get fancy, you can copy the appropriate user picture
from the Accounts preference pane and paste it onto the icon in the
Get Info window of the script. Then you will get the user icon in
QS's first pane instead of a generic Applescript icon.