What specific commands do i need to type (both for the initial setup
and for each use) in order to use the S-Video out (TV out) on my IBM
Thinkpad T42 under a standard installation of the debian-testing
version of Linux?

Some web pages i've seen say, "Hook up the TV cables and reboot your
machine." This is not acceptable to me: I want to be able to activate
TV-out without rebooting.

I want specific commands, like: Run "apt-get install blahtv-2.3.6 ;
blahtv on". Not like: "Install blahtv and activate it".

http://www.stanford.edu/~sanjiv/thinkpad/thinkpad.html These folks are
doing the same thing you are, they found they needed to use a
different driver to solve the problem, There is a thourough walk
through of commands at the site

Yeah, i found that page and the others that turn up when you google
"t42 linux" .. that page in particular doesn't have anything on the
TV-out -- it just talks about installing a different video driver. Or
did i overlook something?

I have yet to find a page explaining what to do to be able to plug my
laptop into a TV (i already have the cables) and make the image on my
screen show up on the TV. And i'm no Linux newbie -- i've been running
it for over five years.

Hya,

Could your try this method :
Try to modify the following section of this file : /etc/X11/XF86Config-4

The section that need to be modified : 
# === Screen Management ===
    Option "DesktopSetup"               "0x00000100" 
    Option "MonitorLayout"              "AUTO, AUTO"
    Option "IgnoreEDID"                 "off"
    Option "HSync2"                     "unspecified" 
    Option "VRefresh2"                  "unspecified" 
    Option "ScreenOverlap"              "0" 

Note the AUTO, AUTO. I also have an additional Screen section:

Section "Screen"
    Identifier   "VGA"
    Device       "ATI Graphics Adapter"
    Monitor      "External Monitor"
    DefaultDepth 24
    Subsection   "Display"
        Depth    24
	Modes    "1600x1200" "1280x1024" "1024x768" "800x600" "640x480"
	ViewPort 0 0
    EndSubsection
EndSection

And the server layout is:

Section "ServerLayout"
    Identifier  "Server Layout"
    Screen "LCD"
    Screen "VGA"
    InputDevice "Mouse1" "CorePointer"
    InputDevice "Keyboard1" "CoreKeyboard"
EndSection

If you start X11 with an external display plugged in, the same image
should appear on both the internal and external display.

Id you plug an external screen when you're working, you would just
need to restart X11 ...