Mail Rule no longer opens new message in Leopard


The following AppleScript (saved as an application bundle without startup screen:

on run
	set theSubject to "subject"
	set theContent to "content"
	tell application "Mail"
		make new outgoing message with properties {subject:theSubject, content:theContent, visible:true}
	end tell
end run

opens a new message window when double-clicked in the Finder or run from Script Editor.

It does NOT open a new message when specified as an AppleScript to be run from a Mail rule.

This same script worked in MacOSX Tiger and does not work in MacOSX Leopard.

Many other people have similar problems when trying to run an AppleScript involving Mail actions from a Mail Rule.
see e.g.
and there are a few posts in MacScripter also that have the same problem.

Can someone help?