Distributed mesh weather sensors
Create a USB weather data collection unit that can be inexpensively produced and provided to (students / schools). Units will be able to log basic meteorological data, "sync" data with other XOs across the mesh, and try to dial the data home to a central server. Users can log actual weather conditions, and learn about how pressure and temperature can be used to predict weather trends. Also, having a large number of data collection units in normally not-monitored-for-this-type-of-stuff places could provide interesting data to researchers.
The first type of data collection we are considering is barometric pressure. That has advantages in that it is easy to collect -- the sensor can be inside, outside, in the sun, in the shade -- it doesn't matter. Moreover, barometric pressure would allow simple predictions of the weather without any communication with the outside world. In a mesh network setting it might be possible to see fronts move by watching the barometric pressure rise/fall throughout a large region.
In addition to the hardware sensor the project would include a software component for logging the data and for comments on the day's weather. Graphs of pressure over time could show how low pressure often coincides with rain.