Wasn't there talk of adding a @media attribute to <video> which could, among other things, hold bitrate information which would allow the UA to auto-determine whether it should play a file?
This would require a change to the current selection algorithm, as the UA now has to make a 'best guess' of which file to use rather than just choosing the first which works, but it's probably worth it. (Plus the other benefits of @media, such as declaring that a particular source has subtitles burned in, etc.) ~TJ