What I normally do to make sure the user gets the latest version of a Flash file (because there is something weird with the caching of Flash content) is to use a serverside script to append the time the file was changed.
I've also been experimenting with setting variables in the swf, although that requires some command line trickery and won't really be practical if you're not using a build script.
Basically you append this to your mxmlc call:
-define+=BUILD::DATE,"2008-12-10"
Then the variable BUILD::DATE will be available during your build, so you can use that to set at static variable which you later read:
private static var COMPILEDATE:String = DATE;
(I wrote all this from memory so it might not be 100% correct syntaxwise)