I think this is the best idea that I’ve seen for distributed computing on the SETI@home model: earthquake detection.
As you probably know, many new laptops come with accelerometers in them which let them do things like park the hard disk heads if dropped, before they hit the ground and the heads crash into large chunks of your data. People have made various fun toy applications using the output of these sensors. But what can you do if you network large numbers of accelerometer-enabled computers together?
Who knows how well it will work, but it’s a great example of lateral thinking. And vertical.