forget about smart dust. what about smart bricks?
these would be devices that can be dropped in any place like a brick, that cost less than a tenner, and can store a few 10s gigabytes of media data and can run computations for a few hours on a battery charge - they have sensor surfaecs (simply light/sound/heat) and they are cheap enough to leave a LOT of them lying around in a city (a bit like velib/free city bikes) and they basically act to hold the city's history. they log what goes on. they can recharge off of intertia or light or ambient heat. they can be your backup server, your neighbourhood watch, your calculator, your e-cache etc etc
much more useful than dust.