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

mdimport -f ~/Library/Mail
doesn't fix that.

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 mcx-disabled -boolean YES
/usr/bin/killall Dock