I have come across this site http://bit.ly/zNjJu1 today and it is everything that I'm trying to achieve. Firstly it's a full screen flash, but secondly you'll notice that even though the movie fills the entire browser, when you re-size a window it doesn't actually scale the movie, it just reveals more of the background imagery, while keeping the content central and original size.
Any ideas would be great?
set your stage scaleMode to noScale and publish for 100%x100%.
but it looks terrible on a 2560x1600 monitor.