In Linux at least, FlashPlayer downloads the video to a temporary file, then plays it from that. At least one version I've used saved the file to /tmp/flash?????? or something like that, and the current version downloads the file to a similar location and immediately deletes it (available by copying the hanging fd in /proc/???/fd for the process running the plugin.) I'm not sure if this is different YouTube versions, different FlashPlayer versions, or different plugin wrappers. Anyway, you might take a look at how the plugin wrappers are written. It's generally been a .flv, .wmv, or .mp4 format.
Kevin Barry
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