For youtube, you need special download tools. Keepvid.com let you do it, at one point. And then Firefox has a plugin called VideoDownloader or something. Then you are stuck with an FLV (Flash Video) formatted video. I don't know what you'd use to convert. FLV quality is so tacky I barely enjoy it on the computer, it would be horrible on the tv.
Yeah as I learned from working on my DIC GI Joe site youtube essentially takes anything and turns it into the following, at best settings (of course using Flash as the method of streaming):
320x240 mpg1/vcd with 56-112 bitrate audio
So at best you would want to try and convert it to a VCD and then it will still look horrid on a TV. I would just weigh the time involved because it can be a long series of steps for not a real worthy outcome (vs watching it on the computer that is)