If you use Remote Apple Events with AppleScript, after performing an upgrade installation (or Archive and Install) of Leopard from an earlier version of Mac OS X, you may notice that your "Remote Apple Events" setting is "off" (the default setting). With AppleScript, Remote Apple Events are extensible mechanisms commonly used in AppleScript for handing interprocess messages used for inter-application communication and also for applications running on different computers connected to a network.
Turn Remote Apple Events back on in Sharing preferences (in System Preferences).
For more information about Sharing settings changing when migrating or performing an Archive and install of Mac OS X 10.5, see this article.