Well guys, the subject tells the story. I've got a Penryn 2.4GHz C2D MacBook Pro with 4GB RAM. I get pretty poor YouTube HD performance in OS X. Normal YouTube is fine, but HD just stutters. I had this problem in both Safari and Firefox, and under Flash 9, and still have it after updating to Flash 10. Internet connection, I believe, is a non-issue, as the stutter persists even if I let the video load fully before playing it. So, any thoughts on this? Its rather frustrating. I'd chalk it up to "Flash sucks in OS X," but I hear of plenty of people with lesser Macs able to run YouTube HD without problem, so I think something else is at issue.