There's a howto disable Spotlight for OS 10.4; for OS 10.5 it's a a bit different, as /etc/hostconfig is about to go away. In short:

$ sudo launchctl unload /System/Library/LaunchDaemons/com.apple.metadata.mds.plist

To disable it permanently, type this instead:
$ sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist

To enable Spotlight type the following and press Return:
$ sudo launchctl load /System/Library/LaunchDaemons/com.apple.metadata.mds.plist

To enable it permanently, type this instead:
$ sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist

Someone else suggested to disable even more:

$ sudo launchctl unload -w /System/Library/LaunchAgents/com.apple.Spotlight.plist
$ sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist

Hm, doing all that makes Spotlight stop indexing, but it's still running, and it respawns upon kill(1)....