Premiere Pro CS6 preview problem: other application shows through
It looks like a problem with the playback engine.  I would try updating your video card drivers and/or making sure the playback engine (in the project/ project settings/general menu) is set to Mercury Playback Engine Software Only.  If that doesn't work, I'd try uninstalling and re-installing Premiere.

I don't know what can cause it on Mac OS, but on Windows seeing something from another application through a portion of a window is generally a sign of a graphics driver being misconfigured or misused by the software in question.