You won't see a video you put up until you publish or post your text though! A little more after fold.
Wow, How nice!! And it's really easy. Exactly as the title reads, thats all you need: two sets of parenthesis, the word youtube, a space, and the goobledygook at the end of the address on the page for the video you're wanting, the YVynnbx1Xsc of the h ttp://www.youtube.com/watch?v=YVynnbx1Xsc.
It's alot easier than this:
<object width="425" height="350"><param name="movie" value="http://www.youtube.com/v/YVynnbx1Xsc"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/YVynnbx1Xsc" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"></embed></object>
which is what YouTube would have you use.
This works in comments as well as in Diaries!
However, when you preview it will still be just the text. The video will replace that when you finally post it.