this might help a bit:
How to convert .flv (flash video) to .avi or .mpg - VideoHelp.com
he's also saying that the video (on some flash player browsers) place the loaded video into your IE temp files, so look there to see if it's available. I've never saved a YouTube file, so I'm not much help, but this guy seems to have found a way to encode your .flv into a playable MPEG