Keith Peters (author of Making Things Move -- see sidebar) has a post today regarding Flex2 and Actionscript 3. His "rant" makes a great point about the differences between a Flex2 application and an Actionscript application.
In short, you have to use the Flex framework to call your program a Flex application. If you code only in Actionscript 3, you've got a Flash application. Makes sense, but I agree with Keith that a lot of people are getting this confused. It confused me initially as well!
Check out Keith's post (err rant) here.