Good catch on exactly what gestures cause the "z" to appear. I am experiencing another effect: A quick short rightward swipe at the very left edge of the trackpad results in Aero Flip 3D displaying all of my open windows in a stack. A leftward swipe from the rightmost edge of the trackpad does...
I am having the same problem. Latest OS (Mac OS X 10.6.7 (10J869)) and Parallels (Build 6.0.11994) with Windows 7 Ultimate 64-bit. Windows Explorer stops and recovers so frequently that Coherence mode is too interrupted to be useful.
