From Mac OS X Hints.com: This script uses Panther’s built in GUI scripting to turn on Internet Connection Sharing in OS X v.10.3.
OS version:
tell application "System Preferences"
  activate
end tell
tell application "System Events"
  tell process "System Preferences"
    click menu item "Sharing" of menu "View" of menu bar 1
    delay 10
    tell window "Sharing"
      if (exists tab group 1) then
      tell tab group 1
        click radio button "Internet"
        delay 1
        click button "Start"
        delay 1
      end tell
      end if
      if exists sheet 1 then
        tell sheet 1
          click button "Start"
        end tell
      end if
    end tell
  end tell
end tell
ignoring application responses
  tell application "System Preferences" to quit
end ignoring