Leopard keeps warning : is an application which was downloaded from the internet. Are you sure you want to open it?

Full thread I found is here , below is the short version on how to remove the extended attributes and possible ACL’s. Leopard adds an ACL called com.apple.quarantine to your download apps :

1. Open the Terminal
2. List the attributes of the app you are having issue with ( in this example , skype )

mac: moti$ xattr -l /Applications/Skype.app
com.apple.quarantine: 0000;49d521d9;Firefox;|org.mozilla.firefox

Remove the “com.apple.quatantine” attribute if it is defined

sudo xattr -d com.apple.quarantine /Applications/Skype.app

3. Check the attributes again

mac: moti$ xattr -l /Applications/Skype.app
mac: moti$

com.apple.quarantine should not be listed.