This is a massive step in the right direction. I remember some time back when the On2 codec was introduced people were asking about H.264 and it the responsed was that is was too CPU intensive, which I guess now must have been resolved.
You will soon (with the release of Flash Player 9 Update 3 Beta 2 be able load and play .mp4,.m4v,.m4a,.mov and .3gp without any chances to your code.
The update als0 includes:
1. Enhancements to full-screen mode to use hardware scaling for improved video performance and quality on systems running Windows 2000 and newer or Mac OS X 10.2 and newer.
2. Faster rendering of vector graphics on multi-core CPUs.
3. Higher quality and performance for downscaling large bitmaps (SWF 9 only).
4. Support for caching common platform components, such as the Flex framework, to reduce average application sizes. This feature is enabled in the Flex 3 beta available on Adobe Labs.
5. Support for full-screen mode on Linux.
Wanna test it out? Well you can as the player is available on labs here.