AppleScript, Keychain and GUI Access from launchd

I wanted to launch some scripts from cron (I know it's deprecated) that would mount some shares using AppleScript (so that share credentials are prompted via GUI or accessed from Keychain) and display notification through Growl. On first try, this worked under Mac OS X 10.5.5, but then began to fail after an upgrade to 10.5.6. I would find error messages that were not very clear: