As root (or prepend the command string with ‘sudo’) enter this command in a terminal window and you’ll be free of the translucent menubar “upgrade” introduced in Leopard (10.5). To reverse it and return to the see-thru bar, just put a 0 in place of the “1” on the end of the string. If you want the sort of gray menubar found in 10.4, use .63 instead of 1 on the end of the line.
defaults write /System/Library/LaunchDaemons/com.apple.WindowServer 'EnvironmentVariables' -dict 'CI_NO_BACKGROUND_IMAGE' 1
You’ll have to reboot to see the change.