Just noticed this, but there is an ActionScript 2 to ActionScript 3Migration reference up on Labs.
This is a great reference to get an overview of some of the changes.
You can view the reference here.
You can download the Flex Builder 2 alpha and start playing around with ActionScript 3 from here.
Note that it looks like a huge document, but a lot of the changes are just APIs moving into a package (as part of the general API clean up).