defaults write com.apple.LaunchServices LSQuarantine -bool NOAfter that, reboot, and you should be set. (If you want to disable quarantine for files already downloaded, you can run this in 10.6 : xattr -d -r com.apple.quarantine ~/Downloads. In 10.5, you need to use this instead: find ~/Downloads -type df -exec xattr -d com.apple.quarantine {}.)
The credit for this tip should really go to Jonathan 'Wolf' Rentzsch, who published it on his Tumblr. He, in turn, credits Ken Aspeslagh with telling him about the defaults write via Twitter.
[robg adds: This should work in 10.5 and 10.6, though I haven't tested it myself.]

