It's not just music videos. I tried to watch some hockey videos tonight, specifically one a friend of mine posted -
http://www.youtube.com/watch?v=CGxcFfLiAH8
Views fine on my computers, but will not play on my iPad 2. For that matter, I can't even search and find it, even if I type the exact name of it, or the User ID.
I am running into a lot of videos doing this recently and I found a google/youtube page updated on 3/19/2011 stating that videos have to be set for syndication to include mobile.
http://www.google.com/support/youtube/bin/answer.py?hl=en&answer=184706