To disable the Spotlight icon in the toolbar in 10.5, simply run:
sudo chmod 0 /System/Library/CoreServices/Spotlight.app
To re-enable it later, do this:
sudo chmod 755 /System/Library/CoreServices/Spotlight.app
[robg adds: I struck out the above about actually disabling Spotlight, as it doesn't work in 10.5. A newer hint will soon be published that will disable Spotlight, and when it shows up, I'll link to it here.]
Update: Here's the new hint.

