There are two reasons why you would want to disable Spotlight indexing. The first being you have another application such as Quicksilver or Launcher (available through the app store) that does the same thing, and more. The other is that you want to temporarly disable indexing because the system is not as responsive as it normal is.
To disable Spotlight indexing:
$ sudo mdutil -a -i off
To re-enable:
$ sudo mdutil -a -i on