This issue remains present in Fusion 5.03 that was released today.
I performed a clean install of OSX 10.8.3 this morning, then installed Fusion 5.03. I upgraded my VM's installed the new tools etc., but the infamous black square box still pops-up as you move the cursor.
If you add mks.noHostCursor = "TRUE" to the VMX...then the entire VM will freeze for about 10 seconds if you attempt to access File Explorer, etc. in Windows 8.
Any help on this would be appreciated.
Dr G