techblog 

If you’ve got Leopard, you may have noticed that it tags files that you download and then warns you that you are opening a downloaded file. This annoys some people, but I think it is quite a nice touch for the average user. As a power user, you might want to remove this tag from the file manually (read: using the command line). To do this you need to use the xattr command as explained nicely by Henrik. It seems like the tag is stored in special extended attributes on the file which show up in detail if you issue the ls command with the -@ flag. To remove them you just do

xattr -d com.apple.quarantine <downloaded_file>

Strangely this command has no man page, but will give usage if you pass it a -h flag.