Disclaimer: My apologies if this idea has been suggested before, I tried to read through most of them before making this suggestion.
Depending on the sensitivity of the accelerometer within the N900, an application could be written to monitor earthquakes. Since the devices won’t be embedded into the ground they could not be accurate to the level of scientific instruments, but the density of monitoring devices would still provide valuable data to seismologists (ie. calculating the relative ground roll amplitude, refining the location of the epicentre [although already very accurate), dense data gridding of initial wave travel times). More importantly this would be a fantastic awareness and educational tool for people to better understand when/where/how often earthquakes happen around the world.
Unfortunately, the monitoring done by any single device would only be possible while it was at rest (not moving while in your pocket or in your hand). So it might be difficult to screen out all the erroneous monitoring (ie. Someone picks up their N900 while monitoring and it registers the movement). But this can be overcome the same way that scientists eliminate the rumbling of trucks near their seismographs, called stacking. Stacking works by taking all the wavelets heard by all the monitoring devices together and summing them. This means that statistically with enough devices you begin to cancel out the random events of the individual devices and compound the events that many devices had in common. An added level of complexity is that you have to make an adjustment for differences in travel time due to the distance from the source (earthquake epicentre).
I imagine the app providing the user with a map of the world showing the location of all the earthquakes that had happened over a few time frames (day, week, year) and their relative strengths. There would also have to be a button to start/stop monitoring. Plus all the data recorded needs to be sent to a central data depository on the web where it can be analysed.
This could provide a tremendous data source for seismologists around the world, and be a fantastic educational tool for earthquake awareness. The uploaded picture is the recording of the 1906 San Francisco Earthquake.
