X-Associated-Content still requires each file to be broken out as its own, doesn't it? I'm thinking more of a single file with a manifest, highly compressed, that only requires one fopen() to push all of the data to the client. Sort of a browser cache warming file. "You need this Javascript, this CSS, and these images for any page on the site. Take this file and you're all set."

Since SPDY multiplexes multiple streams to the same socket, and commonly-used static content will be cached in the server's memory anyway, I don't see much practical difference between what SPDY's X-Associated-Content and what you're proposing.