GPU compositing and surfacing. New drawing API. New text API. Simple 3d for accurate plane rotation. Custom effects with Hydra and AIF (now called Pixel Bender). New File Reference for local file reading. Support for larger bitmaps. New context menu goodies. Something for dynamic sound generation. Yes, Astro is now in public beta.

Demos here. Install here. More information here. See the release notes. This is another good third-party post. As always, Tinic Uro has some more details for the nerds.

Edit: oh f*ck, a new playerglobal.swc is available already (allowing people to compile Flash 10 content with Flex).