I have noticed since Build WebKit r137018 & r137031 Youtube videos are not playing.( I have the latest Flash Plug in) works fine on Chrome, Firefox and Safari ( 6.0.2). I tried to delete the Cookies but didn't work either. I played the videos using HTML5 but it wasn't consistent and i had to keep the cursor on the video to have it play. Thanks.
*** This bug has been marked as a duplicate of bug 104446 ***