The more that I think about varying any of the parts in the block, the more that I think they would just simply break. Changing the size of the piston while in stroke seems extremely hard and "fragile".

I think the technology that BMW is coming out with will be a big step up (no where near changing displacement). They are replacing the cams with actuators that lift the valves. Basically its infinately variable vtec. Prett smart stuff.