I opened a script that was saved as an application in Script Editor in Snow Leopard and it warns:
Document Format is Read Only
This script application is in a format that is no longer supported. You will not be able to save your changes, but you will be able to Save As a new script application. Do you want to edit this file anyway?
If I create a new script application in Snow Leopard will users of 10.5 and 10.4 be able to run it?