When the actual switch in frequencies occurred, the screen briefly flashed. This is generally a symptom of the video memory re-clocking occurring when the frame-buffer is still being scanned out rather than timing the re-clocking to occur during a vblank.
The more glaring issue though when dealing with this re-clocking was that at times there would be visible corruption on the screen when either within the composited Unity desktop or when running OpenGL games.
When attempting to drop the performance level to its lowest state -- as if you wanted to save some power consumption or reduce the heat output -- I tried setting the GT 220 to its lowest state: 135MHz core, 270MHz shader clock, 135MHz memory clock. However, the system in this case would immediately lock-up when attempting the re-clock. Martin Peres says that this is not fully surprising, as support for the particular card is still considered very experimental.