Why is it that the video player on Youtube, and any other site that uses Flash for that sake, clears the entire, current buffer if you seek a point in the stream that hasn't been buffered yet? It's seems pretty stone age not to support chunked buffering on the client side, since downloading and buffering a part of a stream is an "expensive" operation considering the amount of time it usually takes (depending on internet connection of popularity of that video).
Imagine binary-searching a video to find a certain point in the file: Every time you backtrack a little, or jump ahead faster than the download rate, precious data is discarded. I know it's not possible to directly play a video from literally any point in time without decoding the window from the first key frame, or whatever. But because it is possible to only download and play a part of the video/audio stream, i.e. not from the beginning, and play before it's entirely donwloaded, I'd say it's just down to some clever work on the client side to join the downloaded pieces, much like torrents.