The High Quality (HQ) option can be set-up so that every time you log into YouTube you only get HQ. It is under Account>Video Playback Quality>I have a fast connection. Always play higher-quality video when it's available. Hope this helps.
I think the build update in iMovie'08 involves downloading QuickTime 7.4.5. I am sticking with QT 7.4.1 until there is a new QuickTime replacing 7.4.5.