Here are two screenshots of my 512x512x512 generated volume. The one on the left is formed by generating random points in rough bands along the y axis with a higher noise factor towards the top - hopefully representing less dense material. Hidden inside the volume is a cube of much lower density than the surrounding regions.
The one in the centre is the same volume after the 3D Gaussian kernel was applied although it is at a slightly different angle. The right hand side screen capture is using a higher transparency function to show the little hidden cube.
I did run into a problem with the watchdog timer generating a timeout when running the kernel on the full 4kx4kx4k data set. Strangely switching the device to my non-display card didn't help at first. It turns out that Win XP had decided that my second 8800GT had a monitor attached to it, which seems to have activated the watchdog timer for the video driver. Something worth looking out for if you get watchdog timeouts on secondary cards.