I've been experimenting with the video element introduced by HTML 5, in an attempt to replace my current solution that uses Flash.
It soon became evident that the improved performance provided by the native video element came at lack of cross platform compatibility. Videos like m4v files would play nicely on Safari, but wouldn't play on Firefox. Some search on Google led me to http://www.wyodor.net/_Demo/html5/HTML5Video.html, which revealed that Firefox recognizes only video/ogg format.
The next hurdle was how to convert the m4v files to the video ogg format required by Firefox. I found this utility particularly useful for that.