Download the get_flash_videos latest version:
$ git clone https://github.com/monsieurvideo/get-flash-videos.git
Install the dependencies:
# apt-get install perl libmodule-find-perl libwww-mechanize-perl vlc
Install the get_flash_videos script:
$ cd get-flash-videos/
$ make
$ make install
Create a small script that combines these with vlc by adding this to bashrc or a shellscript in PATH the flag -A alsa could be different(when usig pulseaudio it can be omitted):
function ytview {
	if [ "$#" -ne "1" ]; then
		echo "usage $0 yturl"
	else
		get_flash_videos -q -f - $1 | cvlc -A alsa -V aa -
	fi
}
Play a video by typing:
$ ytview http://www.youtube.com/watch?v=QAUzWtLMnU0
Some videos can't be downloaded, but most videos will just work.