Nokia has used ST's accelerometers in the past (at least my 5800 XM that I accidentally drowned in beer had a digital ST accelerometer, pretty much the only component that Google told anything interesting about but can't remember the exact number now), so I wouldn't be surprised to find an ST accelerometer in the N900 as well. ST has done it quite elegantly: they sniff out the capacitance variations between micromachined or etched beams of coated silicon placed perpendicular to each other. Movement causes the masses at the ends of the beams to follow more "slowly", causing a measurable variation in capacitance due to bending of the structures. The datasheet tells you more, just see ST's web pages.