Emmy Huang and Tinic Uro , Justin Everett-Church, and the Flash Dev Center have the Flash Player 9 news with tons of further references.
And it's already being used by a big player; see NewTeeVee's Hulu Adds HD. Beet.TV noted details on the Hulu blog.
The release covers h.264/AAC audio on Windows, Mac, and Linux, and "improved performance through multi-core support for rendering, hardware scaling in full-screen, multi-threaded video decoding, a new algorithm for image scaling, and the Flash Player cache for local caching of common platform components to reduce SWF sizes and app loading times." Also, there's new stuff on Flash Media Server.
Tinic Uro"the moov atom (which is the index information in MPEG-4 files) is at the beginning of the file....you have to wait until the file is completely downloaded before it is played back. You can use tools like qt-faststart.c written by our own Mike Melanson to fix your files so that the index is at the beginning of the file." In Apple tools (or in CS3 with File>Export>QuickTime), you just check the "Fast Start" box.