If your SWF was built with Flash CS3 (aka Flash 9), then both
the Flex SWF and Flash SWF are ActionScript 3 and communication
between them is much easier. However, I have to confess that I
haven't done much with this yet, but I believe the
SWFLoader.content is the equivalent of the Flash SWF's _root
timeline. But I'm not 100% certain of that.