I am going to assume you mean embedded video as opposed to a standard link.
It is done with HTML which is disabled for obvious reasons so simply put you can not.
Edit, I did not realise hunter added the option. Someone may want what follows though.
However most people will have no real objection to visiting youtube, I for one will download the video and then play it back on my machine (FFDShow and media player classic is a far superior solution to the useless flash viewer).
Before the questions roll in:
I use the videodownloader extension for firefox to get my video (it does more than just youtube):
http://javimoya.com/blog/youtube_en.php
I use Gabest's FLV spliiter:
http://sourceforge.net/project/showfiles.php?group_id=82303
And FFDShow to play it back with (I use the one from CCCP for my video), if you use this remember to check FLV1 under codecs:
http://www.cccp-project.net/ (I believe I used the CCCP insurgent to register the splitter too)
If you want to put it in real container I suggest using avisynth's directshowsource (you will need to set fps=something and convertFPS=true), a word to the wise the resolution is usually not mod16 or mod8 (a multiple of 16 or 8 which is required for most codecs and non-RGB colourspaces) by one or two pixels leaving you in a position where addborders or crop is useful.
Audio is a low bitrate MP3, as an example MPEG Audio Layer 3 22050Hz mono 8Kbps which is another good reason to have FFDShow on the case (If you are a having trouble with certain encoders like quenc resample the audio).