
Save the following script (original source) in ~/Library/Scripts/Folder Action Scripts. You'll probably need to create the Scripts directory and its subdirectory, e.g. with mkdir -p ~/Library/Scripts/Folder Action Scripts:
(*
"Unquarantine" by Henrik Nyh <http://henrik.nyh.se/2007/10/lift-the-leopard-download-quarantine>
This Folder Action handler is triggered whenever items are added to the attached folder.
It gets rid of Leopard's annoying "this application was downloaded from the Internet" warnings by stripping the "quarantine" property.
*)
on adding folder items to thisFolder after receiving addedItems
repeat with anItem in addedItems
set anItem's contents to (quoted form of POSIX path of (anItem as alias))
end repeat
set AppleScript's text item delimiters to " "
do shell script "xattr -d com.apple.quarantine " & (addedItems as text)
end adding folder items to
Now navigate to ~/Downloads, or wherever your downloads go. Control-click on the folder in question, then select More » Configure Folder Actions. Check Enable Folder Actions, then Attach the Unquarantine script you saved earlier0 to the folder. And that should be it.
Note that, quite obviously, the folder action only applies to that folder. If you download a file to a directory without this folder action attached, Leopard is free to nag again. (You can read the original version of this post on my blog.)