I get the same thing. I usually try the next video in the list if its what I am looking for and it usually works. Some video's I think just dont work on a bb.
I also have encountered that problem even had it happen on the same vid if i try to play it a second time what i have done is go option in browser, cache operations and cleared it all out then go back to the youtube page and it played the video.
I have the same problem as everyone. I don't think it's your phone. I think the BB is just unreliable when it comes to playing video. Another site you might try is "vuclip" it seems to work a little better than you tube on my phone.