alt Hacker NewsRFID tags are powered wirelessly, one could imagine powering smaller particles when operating on higher frequencies (RFID is on 13.something MHz requiring relatively large coils). A directional antenna could send a pulsed beam to power a subset of the particles in the area and afterwards receive their signals.
It needs to be in the infrared spectrum at least to be useful for smart dust, otherwise the package size is still dominated by the size of the antenna. Even mm-wave radar is marginal here.