It appears that when embedding Quicktime in a webpage, being viewed by Safari 4 in Snow Leopard, Quicktime no longer passes cookies to the server. So, if you’re having the Quicktime plugin load a file that uses cookie data to verify permissions, you’ll need to move to a query string model.
This only happens when Safari is running in 64bit mode, so I imagine it has to do with the “plugins running as separate entities” crash protection that Snow Leopard adds.
This does not appear to impact the Flash plugin.