And AFAIK is Flash IDE Oldschool, Flex is the ne Way.
@joshg: #include and AS2/3 (OOP) ?! No way!
The use of #include I was referring to is specifically for within the Flash IDE, so that you're saving all your code to text files that can be managed properly with code repository tools. ie. So that your code isn't scattered across a binary .fla file that you can't run diff tools on.
If you're not using the Flash IDE, then never mind I guess.
I don't see why the Flash IDE is "oldschool". It's not the usual programmer's environment, but it has a lot of advantages once you get the hang of it. The timeline can be a powerful tool for organizing your art assets and handling things like different game modes, menu screens, etc.
The only reason I could see that Flex seems to get this aura of being newer and better is that it supported AS3 before Flash itself did, but Flash CS3 has caught up to it anyway.
That said, there's nothing wrong with Flex either, and it's cheaper (still free I suppose?), so if that floats your boat then go for it.