So far, for me, the graphical glitch was only apparent on the menu bar. When my logic board went on my G5 iMac the problem then was widespread across the screen. So I'm hoping that indicates more a bug within Lion than a logic board problem.
I don't remember if I was using flash, QT or YouTube at the time but I have experienced the issues mentioned elsewhere whereby the system becomes unresponsive apart from the mouse cursor moving, only then would a hard reboot remedy the situation.
Hopefully 10.7.1 will iron out these creases.