Joy! ocdinsomniac has found a way to disable Spotlight but still allow searches in Finder.
Disabling Spotlight completely in /etc/hostconfig was effective, but you couldn't search by filename in Finder windows any more. Use these two commands instead (and remember to run them again if you attach a new drive):
sudo mdutil -i off / /Volumes/*
sudo mdutil -E / /Volumes/*
Still can't search inside Mail messages though ... even
doesn't fix that.
mdimport -f ~/Library/Mail
There's a separate hint to get rid of the Spotlight icon from the toolbar, and another pair of commands for disabling Dashboard:
/usr/bin/defaults write com.apple.dashboard mcx-disabled -boolean YES