NB: I have archived the images on this page to save disk space. You are welcome to them, however; email me and I'll restore them.
Pavle came to me with this idea, high-resolution images for which I generated. Consider many chips placed at one point near the center. It will relax to something like this (generated with xsand's auto-double feature):
(The circle has diameter 249.) Now, take the all-ones state and add the state above to it. You get this:
(The octagon has diameter 319.) Try the same experiment with the all-twos state. (I had to do it on a bigger grid, but the starting state is the same):
(The square has diameter 523.) On an all-threes state, we get firings that go off to infinity.
So we go from a circle to an octagon to a square to infinity. It would be interesting to see what patterns the firings make, as well as what sort of images we would see on a grid of equilateral triangles.
There is also a picture of many more chips at the center, but a few spilled off the edges before I stopped it:
