If you have any of the special "cube" effects enabled in your UI, VNC may not work properly.  In

If you have any of the special "cube" effects enabled in your UI, VNC may not work properly. In my case it didn't work at all. Try connecting after you disable all the effects and reboot the machine.

Also, you must have a Gnome session going in order to access VNC. What this means is you have to log in on the actual computer and start a desktop session in order to connect via VNC. Hope this helps somewhat...