Is this possible? I would like to have a main swf that can be viewed by users who either have flash 9 or flash 10 installed. Inside of this swf, a separate module will be loaded only for users who have flash 10 installed. This module will use some advanced flash 10 features.
Sure, this should work fine just as you've described it. Make your base SWF in Flash 9, do a version check, and optionally load in another SWF published as 10. Flash will render the advanced features in the v10 SWF, regardless of the version of the SWF it was loaded into.
Just remember that the version is returned as a String, so be sure to do a parseInt on it.