Comment 16 for bug 729307

Niklas: this thread (http://ubuntuforums.org/showthread.php?t=1698956) also discovered the cookie issue.
Apparently, the cookie PREF, with a value of 'fv=<some version of flash>' is at fault. if you prevent this cookie from being set, or manually set it to an older version of flash and prevent it from being overwritten, the videos will play normally.

The speculation is that youtube uses the value of this cookie to determine what format stream to send out, Perhaps youtube is supporting some new feature in 10.2, which doesn't work correctly on Ubuntu. So, if you tell youtube to send the stream for an older version (even if you have 10.2), it works correctly. That's the speculation, at least