If you want to watch YouTube video on the plane (and Google TechChannel has some amazing collection of videos), here is how to save it to disk, without any tools or third party program, using just your Mac and Safari out of the box.
Go to YouTube page and select video you are interested in in Safari. While playing, press Option-Command-A which opens Activity window. Locate the video stream - it will be the largest item in the list. Double click it, which causes Safari to download it.
After finish, go to download folder, locate the file (usually named get_video) and rename it to SOMETHING.flv. Open with any FLV enabled player - VLC works like charm.
At the end usual disclaimer: you should always make sure you do not violate copyright law or law of the country you reside in. I am not recommending or endorsing any behaviour that would cause violation of copyright law, merely describing interesting technical feature of a popular browser ;-).