Think I'm crazy in suggesting in suggesting that a machine could track every single plant over an area of several acres? This Digg item says this experimental camera can image "every single blade of grass" over an area of "four football fields" in one shot.
Granted that they're using chemical film in the camera itself, but that film is scanned as soon as it's processed, and all subsequent image manipulation is done digitally. The file resulting from a full-resolution scan of one such film is 24 GB, which does push the limits of current technology a bit.
While really not at all the same as what a cultibot would do in cataloging all of the plants within the area it tended, this does suggest that the level of complexity involved is within the reach of either current technology or what's just around the corner. Maybe the database for a five-acre plot would occupy a few terabytes, but even that isn't unmanageable.