it is a screen tearing issue, due to no vsync support in hildon-desktop, due in part to PowerVR's closed drivers, and Nokia flagging this bug as a 'WILL NOT FIX'. It may happen in PR1.3, but it is highly unlikely. MeeGo adaptations are slated to fix this. As it is, there is no way around it. You can minimize the annoyance of this by overclocking your device, optimizing your cache pressure/swappiness, and using a custom transitions.ini. All of this can be found in the wiki.