Starting in Mac OS X Leopard, you may have noticed that when you download a file from the web and go to open it, you will get a prompt saying something along these lines.
This is Mac OS X just being safe, but if you always know for sure what you’re downloading it could even be overly safe. You can disable this quarantine message by launching the Mac Terminal and typing the following command:
defaults write com.apple.LaunchServices LSQuarantine -bool NO
You will then need to reboot (although killing the Finder should work as well) for the changes to take effect. To reverse this and get the file quarantine message back, type:
defaults write com.apple.LaunchServices LSQuarantine -bool YES
Again you will need to reboot (or kill the Finder) to have the changes back.