BEST. HINT. EVER.
macosxhints.com - 10.5: Get rid of the translucent menu bar:
One down, one to go? Steve Miner somehow managed to figure out how to disable the translucent menu bar. He posted a solution that involved editing a system-level plist, and then commenter Krioni came up with a one-line Terminal command.
sudo defaults write /System/Library/LaunchDaemons/com.apple.WindowServer 'EnvironmentVariables' -dict 'CI_NO_BACKGROUND_IMAGE' 0.63