Silverlight also uses the browser cache. I'm surprised that Flash doesn't make use of it (if I had to guess I'd say it does, or rather at least it can).

Flash has used the browser cache since the inception of the plugin. In fact it routes all http requests through the browser itself, it has no built-in stack of it's own. (unless you're on the desktop)

SL uses the browser stack by default, but also lets you use its own HTTP client stack.