Hardware:
Acer Aspire 7560G, A8-3500M - Radeon HD 6620G, external display attached via HDMI.

Software:
That laptop is supported since Linux 3.2, so because of that I use it for four months with Kubuntu 11.10, Linux 3.2, Xorg Edgers PPA, KDE 4.7. Now I upgrade to Kubuntu 12.04 with Linux 3.2, Mesa 8.0.1, KDE 4.8 and issue almost the same (but flickering is less often than before).

Steps to reproduce:
In November and December (with latest at this moment R600g from Xorg Edgers PPA and KDE 4.7)  issue is reproduceable even when I click to Kickoff or minimize/maximize apps (it still happen sometimes but very rare). Now (with default driver in Kubuntu 12.04 and KDE 4.8) steps to reproduce is a little different but that still easy - flickering is reproduceable when KWin playback animation of "carousel", "desktop cube", "slide back" (and probably many others) effects.

How that black flickering looks like:
It's looks like black rectangle/polygon that cover 1/3 of display in the middle or bottom.

Additinal information:
Discrete GPU is disabled on every boot by script. Usually I use DynPM power profile for GPU in APU, but that probably unrelated to this issue, because with default profile issue looks the same.
Issue is reproduceable only with OpenGL version of KWin. There is no black flickering on LVDS with OpenGL ES version of KWin. Issue is also visible only on LVDS, never on external display that attached to HDMI (that selected as main in KDE display settings). Issue is also reproduceable when external display is not attached.
There is no black flickering on LVDS with fglrx.