If you want to play a youtube video on your cellular you may need to convert it to the format you cell phone can actually play, I have tested this process with a Sony Ericsson K550i and it worked great, I suppose it should work on almost all modern cellphones.
First thing you need is to download the video from youtube, there are a lot of ways to do that, more than a year ago, I have written this Howto download a Youtube video with wget, but you can now also use some other online tools like, Savevid, check this youtube video to know how to use the page.
Ok, once you have it downloaded in your disk proceed to convert it to a format your cellphone can play, for that we are going to use
First install the application
sudo aptitude install ffmpeg
Now use it in this format:
ffmpeg -i video.mp4 -s 176x144 -vcodec h263 -r 25 -b 12200 -ab 12200 -ac 1 -ar 8000 arma.3gp
The video I have used for this example is this one.
Armageddon Tribute, just cause is one of my daughters favorites.
Just be sure to change the names video.mp4 and arma.3gp accordingly but keep the .3gp extension on the destination.